Functions and their properties

Function is one of the most important mathematical concepts.Function They call such a dependence of the variable y on the variable x in which each value of the variable x corresponds to a single value of the variable y.

Variable X called independent variable or argument. Variable at called dependent variable. They also say thatthe variable y is a function of the variable x. The values ​​of the dependent variable are calledfunction values.

If the dependence of the variableat from variableX is a function, then it can be written briefly as follows:y= f( x ). (Read:at equalsf fromX .) Symbolf( x) denote the value of the function corresponding to the value of the argument equal toX .

All values ​​of the independent variable formdomain of a function . All values ​​that the dependent variable takes formfunction range .

If a function is specified by a formula and its domain of definition is not specified, then the domain of definition of the function is considered to consist of all values ​​of the argument for which the formula makes sense.

Methods for specifying a function:

1.analytical method (the function is specified using a mathematical formula;

2.tabular method (the function is specified using a table)

3.descriptive method (the function is specified by verbal description)

4. graphical method (the function is specified using a graph).

Function graph name the set of all points of the coordinate plane, the abscissas of which are equal to the values ​​of the argument, and the ordinates - corresponding function values.

BASIC PROPERTIES OF FUNCTIONS

1. Function zeros

Zero of a function is the value of the argument at which the value of the function is equal to zero.

2. Intervals of constant sign of a function

Intervals of constant sign of a function are sets of argument values ​​on which the function values ​​are only positive or only negative.

3. Increasing (decreasing) function.

Increasing in some interval a function is a function for which higher value the argument from this interval corresponds to a larger value of the function.

Function y = f ( x ) called increasing on the interval (A; b ), if for any x 1 And x 2 from this interval such thatx 1 < x 2 , inequality is truef ( x 1 )< f ( x 2 ).

Descending in a certain interval, a function is a function for which a larger value of the argument from this interval corresponds to a smaller value of the function.

Function at = f ( x ) called decreasing on the interval (A; b ) , if for any x 1 And x 2 from this interval such that x 1 < x 2 , inequality is truef ( x 1 )> f ( x 2 ).

4. Even (odd) function

Even function - a function whose domain of definition is symmetrical with respect to the origin and for anyX from the domain of definition the equalityf (- x ) = f ( x ) . The graph of an even function is symmetrical about the ordinate.

For example, y = x 2 - even function.

Odd function- a function whose domain of definition is symmetrical with respect to the origin and for any X from the domain of definition the equality is true f (- x ) = - f (x ). The graph of an odd function is symmetrical about the origin.

For example: y = x 3 - odd function .

A function of general form is not even or odd (y = x 2 +x ).

Properties of some functions and their graphics

1. Linear function called a function of the form , Where k And b – numbers.

Domain linear function- a bunch ofR real numbers.

Graph of a linear functionat = kx + b ( k ≠ 0) is a straight line passing through the point (0;b ) and parallel to the lineat = kx .

Straight, not parallel to the axisOU, is the graph of a linear function.

Properties of a linear function.

1. When k > 0 function at = kx + b

2. When k < 0 function y = kx + b decreasing in the domain of definition.

y = kx + b ( k ≠ 0 ) is the entire number line, i.e. a bunch ofR real numbers.

At k = 0 set of function valuesy = kx + b consists of one numberb .

3. When b = 0 and k = 0 the function is neither even nor odd.

At k = 0 linear function has the formy = b and at b ≠ 0 it is even.

At k = 0 and b = 0 linear function has the formy = 0 and is both even and odd.

Graph of a linear functiony = b is a straight line passing through the point (0; b ) and parallel to the axisOh. Note that when b = 0 function graphy = b coincide with the axis Oh .

5. When k > 0 we have that at> 0, if and at< 0 if . At k < 0 we have that y > 0 if and at< 0, если .

2. Function y = x 2

Rreal numbers.

Giving a variableX several values ​​from the function's domain and calculating the corresponding valuesat according to the formula y = x 2 , we depict the graph of the function.

Graph of a function y = x 2 called parabola.

Properties of the function y = x 2 .

1. If X= 0, then y = 0, i.e. a parabola has coordinate axes common point(0; 0) - origin.

2. If x ≠ 0 , That at > 0, i.e. all points of the parabola, except the origin, lie above the x-axis.

3. Set of function valuesat = X 2 is the span functionat = X 2 decreases.

X

3.Function

The domain of this function is the span functiony = | x | decreases.

7. Lowest value function takes at pointX, it equals 0. Greatest value does not exist.

6. Function

Function scope: .

Function range: .

The graph is a hyperbole.

1. Function zeros.

y ≠ 0, no zeros.

2. Intervals of constancy of signs,

If k > 0, then at> 0 at X > 0; at < 0 при X < О.

If k < 0, то at < 0 при X > 0; at> 0 at X < 0.

3. Intervals of increasing and decreasing.

If k > 0, then the function decreases as .

If k < 0, то функция возрастает при .

4. Even (odd) function.

The function is odd.

Square trinomial

Equation of the form ax 2 + bx + c = 0, where a , b And With - some numbers, anda≠ 0, called square.

In a quadratic equationax 2 + bx + c = 0 coefficient A called the first coefficient b - second coefficients, with - free member.

Root formula quadratic equation has the form:

.

The expression is called discriminant quadratic equation and is denoted byD .

If D = 0, then there is only one number that satisfies the equation ax 2 + bx + c = 0. However, we agreed to say that in this case the quadratic equation has two equal real roots, and the number itself called double root.

If D < 0, то квадратное уравнение не имеет действительных корней.

If D > 0, then the quadratic equation has two different real roots.

Let a quadratic equation be givenax 2 + bx + c = 0. Since a≠ 0, then dividing both parts given equation onA, we get the equation . Believing And , we arrive at the equation , in which the first coefficient is equal to 1. This equation is calledgiven.

The formula for the roots of the above quadratic equation is:

.

Equations of the form

A x 2 + bx = 0, ax 2 + s = 0, A x 2 = 0

are called incomplete quadratic equations. Incomplete quadratic equations are solved by factoring the left side of the equation.

Vieta's theorem .

The sum of the roots of a quadratic equation is equal to the ratio of the second coefficient to the first, taken with the opposite sign, and the product of the roots is the ratio of the free term to the first coefficient, i.e.

Converse theorem.

If the sum of any two numbersX 1 And X 2 equal to , and their product is equal, then these numbers are the roots of the quadratic equationOh 2 + b x + c = 0.

Function of the form Oh 2 + b x + c called square trinomial. The roots of this function are the roots of the corresponding quadratic equationOh 2 + b x + c = 0.

If the discriminant of a quadratic trinomial is greater than zero, then this trinomial can be represented as:

Oh 2 + b x + c = a(x-x 1 )(x-x 2 )

Where X 1 And X 2 - roots of the trinomial

If the discriminant of a quadratic trinomial is zero, then this trinomial can be represented as:

Oh 2 + b x + c = a(x-x 1 ) 2

Where X 1 - the root of the trinomial.

For example, 3x 2 - 12x + 12 = 3(x - 2) 2 .

Equation of the form Oh 4 + b X 2 + s= 0 is called biquadratic. Using variable replacement using the formulaX 2 = y it reduces to a quadratic equationA y 2 + by + c = 0.

Quadratic function

Quadratic function is a function that can be written by a formula of the formy = ax 2 + bx + c , Where x – independent variable,a , b And c – some numbers, anda ≠ 0.

The properties of the function and the type of its graph are determined mainly by the values ​​of the coefficienta and discriminant.

Properties of a quadratic function

Domain:R;

Range of values:

at A > 0 [- D/(4 a); ∞)

at A < 0 (-∞; - D/(4 a)];

Even, odd:

at b = 0 even function

at b ≠ 0 function is neither even nor odd

at D> 0 two zeros: ,

at D= 0 one zero:

at D < 0 нулей нет

Sign constancy intervals:

if a > 0, D> 0, then

if a > 0, D= 0, then

e if a > 0, D < 0, то

if a< 0, D> 0, then

if a< 0, D= 0, then

if a< 0, D < 0, то

- Intervals of monotony

for a > 0

at a< 0

The graph of a quadratic function isparabola – a curve symmetrical about a straight line , passing through the vertex of the parabola (the vertex of the parabola is the point of intersection of the parabola with the axis of symmetry).

To graph a quadratic function, you need:

1) find the coordinates of the vertex of the parabola and mark it in the coordinate plane;

2) construct several more points belonging to the parabola;

3) connect the marked points with a smooth line.

The coordinates of the vertex of the parabola are determined by the formulas:

; .

Converting function graphs

1. Stretching graphic artsy = x 2 along the axisat V|a| times (at|a| < 1 is a compression of 1/|a| once).

If, and< 0, произвести, кроме того, зеркальное отражение графика отно­сительно оси X (the branches of the parabola will be directed downwards).

Result: graph of a functiony = ah 2 .

2. Parallel transfer function graphicsy = ah 2 along the axisX on| m | (to the right when

m > 0 and to the left whenT< 0).

Result: function graphy = a(x - t) 2 .

3. Parallel transfer function graphics along the axisat on| n | (up atp> 0 and down atP< 0).

Result: function graphy = a(x - t) 2 + p.

Quadratic inequalities

Inequalities of the formOh 2 + b x + c > 0 andOh 2 + bx + c< 0, whereX - variable,a , b AndWith - some numbers, anda≠ 0 are called inequalities of the second degree with one variable.

Solving a second degree inequality in one variable can be thought of as finding the intervals in which the corresponding quadratic function takes positive or negative values.

To solve inequalities of the formOh 2 + bx + c > 0 andOh 2 + bx + c< 0 proceed as follows:

1) find the discriminant of the quadratic trinomial and find out whether the trinomial has roots;

2) if the trinomial has roots, then mark them on the axisX and through the marked points a parabola is drawn schematically, the branches of which are directed upward atA > 0 or down whenA< 0; if the trinomial has no roots, then schematically depict a parabola located in the upper half-plane atA > 0 or lower atA < 0;

3) found on the axisX intervals for which the points of the parabola are located above the axisX (if inequality is solvedOh 2 + bx + c > 0) or below the axisX (if inequality is solvedOh 2 + bx + c < 0).

Example:

Let's solve the inequality .

Consider the function

Its graph is a parabola, the branches of which are directed downward (since ).

Let's find out how the graph is located relative to the axisX. Let's solve the equation for this . We get thatx = 4. The equation has a single root. This means that the parabola touches the axisX.

By schematically depicting a parabola, we find that the function takes negative values ​​for anyX, except 4.

The answer can be written like this:X - any number not equal to 4.

Solving inequalities using the interval method

solution diagram

1. Find zeros function on the left side of the inequality.

2. Mark the position of the zeros on the number axis and determine their multiplicity (Ifk i is even, then zero is of even multiplicity ifk i odd is odd).

3. Find the signs of the function in the intervals between its zeros, starting from the rightmost interval: in this interval the function on the left side of the inequality is always positive for the given form of inequalities. When moving from right to left through the zero of a function from one interval to an adjacent one, one should take into account:

if zero is odd multiplicity, the sign of the function changes,

if zero is even multiplicity, the sign of the function is preserved.

4. Write down the answer.

Example:

(x + 6) (x + 1) (X - 4) < 0.

Function zeros found. They are equal:X 1 = -6; X 2 = -1; X 3 = 4.

Let us mark the zeros of the function on the coordinate linef ( x ) = (x + 6) (x + 1) (X - 4).

Let's find the signs of this function in each of the intervals (-∞; -6), (-6; -1), (-1; 4) and

It is clear from the figure that the set of solutions to the inequality is the union of the intervals (-∞; -6) and (-1; 4).

Answer: (-∞ ; -6) and (-1; 4).

The considered method for solving inequalities is calledinterval method.

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The function y=x^2 is called a quadratic function. The graph of a quadratic function is a parabola. General form The parabola is shown in the figure below.

Quadratic function

Fig 1. General view of the parabola

As can be seen from the graph, it is symmetrical about the Oy axis. The Oy axis is called the axis of symmetry of the parabola. This means that if you draw a straight line on the graph parallel to the Ox axis above this axis. Then it will intersect the parabola at two points. The distance from these points to the Oy axis will be the same.

The axis of symmetry divides the graph of a parabola into two parts. These parts are called branches of the parabola. And the point of a parabola that lies on the axis of symmetry is called the vertex of the parabola. That is, the axis of symmetry passes through the vertex of the parabola. The coordinates of this point are (0;0).

Basic properties of a quadratic function

1. At x =0, y=0, and y>0 at x0

2. The quadratic function reaches its minimum value at its vertex. Ymin at x=0; It should also be noted that the function does not have a maximum value.

3. The function decreases on the interval (-∞;0] and increases on the interval)