Area L

Let $f(x)$ be a continuous increasing function in the interval $a\leq x\leq b$ where $0 < a < b$ and $0\leq f(a) < f(b)$.

Can you prove the following formula with the help of a sketch? $$\int_{f(a)}^{f(b)} f^{-1}(t) \,dt + \int_a^bf(x) \,dx = bf(b) - af(a).$$

Why must $f(x)$ be increasing in the interval $a\leq x\leq b$?

Once you've proved the formula, find the value of $\int _1^4 \sqrt t \,dt$, in two different ways; firstly by evaluating the integral directly, and secondly by using the formula above with $f(x)=x^2$.