What is the integral from 0 to pi/4 of cos^3(2x) ?

The integral from 0 to pi/4 of cos^3(2x) is 1/3

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integral from 0 to pi/4 of cos^3(2x)

\int_{0}^{\frac{π}{4}} cos^{3} (2 x) d x

\frac{1}{3}

Decimal

0.33333 \dots

Solution steps

\int_{0}^{\frac{π}{4}} cos^{3} (2 x) d x

Apply u-substitution

= \int_{0}^{\frac{π}{2}} cos^{3} (u) \frac{1}{2} d u

Take the constant out:

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

= \frac{1}{2} \cdot \int_{0}^{\frac{π}{2}} cos^{3} (u) d u

Rewrite using trig identities

= \frac{1}{2} \cdot \int_{0}^{\frac{π}{2}} (1 - sin^{2} (u)) cos (u) d u

Apply u-substitution

= \frac{1}{2} \cdot \int_{0}^{1} 1 - v^{2} d v

Apply the Sum Rule:

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

= \frac{1}{2} (\int_{0}^{1} 1 d v - \int_{0}^{1} v^{2} d v)

\int_{0}^{1} 1 d v = 1

\int_{0}^{1} v^{2} d v = \frac{1}{3}

= \frac{1}{2} (1 - \frac{1}{3})

Simplify

\frac{1}{2} (1 - \frac{1}{3}) : \frac{1}{3}

= \frac{1}{3}

What is the integral from 0 to pi/4 of cos^3(2x) ?
The integral from 0 to pi/4 of cos^3(2x) is 1/3