Copyright © University of Cambridge. All rights reserved.

## 'Area L' printed from http://nrich.maths.org/

Let $f(x)$ be a continuous increasing function in the interval $a\leq x\leq b$ where $0 < a < b$ and $0\leq f(a) < f(b)$. Prove the following formula: $$\int_{f(a)}^{f(b)} f^{-1}(t)dt + \int_a^bf(x)dx = bf(b) - af(a).$$

Find the value of $\int _1^4 \sqrt t dt$, firstly by evaluating the integral directly, and secondly by using the formula above with $f(x)=x^2$.

Use the formula to evaluate $\int_0^1\sin^{-1}t dt.$

What other functions can you integrate more easily using this formula than by other means? Why must $f(x)$ be increasing in the interval $a\leq x\leq b$? How could you evaluate a similar integral if $f(x)$ is decreasing?