Question

Evaluate the integral

Answer 1

Hello, please consider the following.

$\displaystyle \begin{aligned} \int\limits^x {5sin(5t)sin(t)} \, dt &= -\int\limits^x {5sin(5t)} \, d(cos(t))\\ \\&=-[5sin(5t)cos(t)]+ \int\limits^x {25cos(5t)cos(t)} \, dt\\\\&=-5sin(5x)cos(x)+ \int\limits^x {25cos(5t)} \, d(sin(t))\\ \\&=-5sin(5x)cos(x)+[25cos(5t)sin(t)]+ \int\limits^x {25sin(5t)sin(t)} \, dt\\\\&=-5sin(5x)cos(x)+25cos(5x)sin(x)+ \int\limits^x {(25*5)sin(5t)sin(t)} \, dt\end{aligned}$

And we can recognise the same integral, so.

$\displaystyle (25-1)\int\limits^x {5sin(5t)sin(t)} \, dt= +5sin(5x)cos(x)-25cos(5x)sin(x)$

And then,

$\displaystyle \Large \boxed{\sf \bf\int\limits^x {5sin(5t)sin(t)} \, dt=\dfrac{5sin(5x)cos(x)-25cos(5x)sin(x)}{24}+C}$

Thanks