Consider: I) If $f(x)$ is bounded for $x\in[a,b]$, then area bounded by $y=f(x)$, $x$-axis, $x=a$ and $x=b$ is $\int_a^b f(x)dx$. II) If $f(x)$ is bounded and differentiable on $[a,b]$, then the area between $y=f(x)$ and $y=f^{-1}(x)$ is equal to double the value of $\int_a^b|f(x)-x|dx$.