Сколько чисел от 15 до 123 кратно 7?

21, 28, 35, 42, 49, 56, 63, 70, 77, 84, 91, 98, 105, 112, 119

Свойства функции y=cosx

1. Область определения — все действительные числа (множество R).

2. Множество значений — промежуток [−1;1].

3. Функция y=cosx имеет период 2π.

4. Функция y=cosx является чётной.

5. Нули функции: x=π2+πn,n∈Z;

наибольшее значение равно 1 при x=2πn,n∈Z;

наименьшее значение равно −1 при x=π+2πn,n∈Z;

значения функции положительны на интервале (−π2;π2), с учётом периодичности функции на интервалах (−π2+2πn;π2+2πn),n∈Z;

значения функции отрицательны на интервале (π2;3π2), с учётом периодичности функции на интервалах (π2+2πn;3π2+2πn),n∈Z.

6. Функция y=cosx:

- возрастает на отрезке [π;2π], с учётом периодичности функции на отрезках [π+2πn;2π+2πn],n∈Z;

- убывает на отрезке [0;π], с учётом периодичности функции на отрезках [2πn;π+2πn],n∈Z.

Simplifying

4x3 + -81x = 0

Reorder the terms:

-81x + 4x3 = 0

Solving

-81x + 4x3 = 0

Solving for variable 'x'.

Factor out the Greatest Common Factor (GCF), 'x'.

x(-81 + 4x2) = 0

Factor a difference between two squares.

x((9 + 2x)(-9 + 2x)) = 0

Subproblem 1

Set the factor 'x' equal to zero and attempt to solve:

Simplifying

x = 0

Solving

x = 0

Move all terms containing x to the left, all other terms to the right.

Simplifying

x = 0

Subproblem 2

Set the factor '(9 + 2x)' equal to zero and attempt to solve:

Simplifying

9 + 2x = 0

Solving

9 + 2x = 0

Move all terms containing x to the left, all other terms to the right.

Add '-9' to each side of the equation.

9 + -9 + 2x = 0 + -9

Combine like terms: 9 + -9 = 0

0 + 2x = 0 + -9

2x = 0 + -9

Combine like terms: 0 + -9 = -9

2x = -9

Divide each side by '2'.

x = -4.5

Simplifying

x = -4.5

Subproblem 3

Set the factor '(-9 + 2x)' equal to zero and attempt to solve:

Simplifying

-9 + 2x = 0

Solving

-9 + 2x = 0

Move all terms containing x to the left, all other terms to the right.

Add '9' to each side of the equation.

-9 + 9 + 2x = 0 + 9

Combine like terms: -9 + 9 = 0

0 + 2x = 0 + 9

2x = 0 + 9

Combine like terms: 0 + 9 = 9

2x = 9

Divide each side by '2'.

x = 4.5

Simplifying

x = 4.5

Solution

x = {0, -4.5, 4.5}