What is the integral of 4cos^2(9x) ?

The integral of 4cos^2(9x) is 2(x+1/18 sin(18x))+C

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integral of 4cos^2(9x)

\int 4 cos^{2} (9 x) d x

2 (x + \frac{1}{18} sin (18 x)) + C

Solution steps

\int 4 cos^{2} (9 x) d x

Take the constant out:

\int a \cdot f (x) d x = a \cdot \int f (x) d x

= 4 \cdot \int cos^{2} (9 x) d x

Rewrite using trig identities

= 4 \cdot \int \frac{1}{2} (1 + cos (18 x)) d x

Take the constant out:

\int a \cdot f (x) d x = a \cdot \int f (x) d x

= 4 \cdot \frac{1}{2} \cdot \int 1 + cos (18 x) d x

Apply the Sum Rule:

\int f (x) \pm g (x) d x = \int f (x) d x \pm \int g (x) d x

= 4 \cdot \frac{1}{2} (\int 1 d x + \int cos (18 x) d x)

\int 1 d x = x

\int cos (18 x) d x = \frac{1}{18} sin (18 x)

= 4 \cdot \frac{1}{2} (x + \frac{1}{18} sin (18 x))

Simplify

4 \cdot \frac{1}{2} (x + \frac{1}{18} sin (18 x)) : 2 (x + \frac{1}{18} sin (18 x))

= 2 (x + \frac{1}{18} sin (18 x))

Add a constant to the solution

= 2 (x + \frac{1}{18} sin (18 x)) + C

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What is the integral of 4cos^2(9x) ?
The integral of 4cos^2(9x) is 2(x+1/18 sin(18x))+C