91Ó°ÊÓ

${∫}_a^b{\sin }^{2}kxdx={∫}_a^b{\cos }^{2}kxdx=\frac{1}{2}(b-a)$If $k(b-a)$ is an integral multiples of $\mathrm{π}/2$.

The average value of a function over a particular interval can be found with an expression involving an integral.

Let's say that interval is $\mathbf{[}\mathit{a}\mathbf{,}\mathit{b}\mathbf{|}$.

Then the average value of f(x) over said interval is $\frac{\mathbf{1}}{\mathbf{b}\mathbf{-}\mathbf{a}}{\mathbf{∫}}_{\mathbf{a}}^{\mathbf{b}}\mathit{f}\mathbf{\left(}\mathit{x}\mathbf{\right)}\mathit{d}\mathit{x}$.

To show that:

${∫}_a^b{\sin }^{2}kxdx={∫}_a^b{\cos }^{2}kxdx=\frac{1}{2}(b-a)$ if $k(b-a)$is an integral multiple of $\mathrm{π}/2$.

${∫}_a^b{\sin }^{2}kxdx+{∫}_a^b{\cos }^{2}dx={∫}_a^{b}dx=b-a$ ...... (1)

With the help of average value method calculate as shown below.

$$\begin{gathered} {∫}_a^b{\sin }^2\left(kx\right)dx=\frac{1}{k}{∫}_{ka}^{kb}{\sin }^{2}tdt \\ =\frac{1}{2k}{∫}_{ak}^{bk}(1-\cos (2t\left)\right)dt \\ =\frac{1}{2k}\left[k(b-a)-{\left.\frac{1}{2}\sin \left(2t\right)\right|}_{ak}^{bk}\right] \end{gathered}$$

$=\frac{1}{2}(b-a)$ ...... (2)

The second equation disappears since$ka=n\mathrm{π}/2$ and$kb=n\mathrm{π}/2,n∈\mathrm{ℤ}$.

Use equation first and second:

${∫}_a^b{\cos }^{2}dx=\frac{1}{2}(b-a)$

Thus, the solution is ${∫}_a^b{\sin }^{2}kxdx={∫}_a^b{\cos }^{2}kxdx=\frac{1}{2}(b-a)$.