Show that if 0 (x) 1 for all x, then the arc length of y

Show that if 0 ≤ ƒ(x) ≤ 1 for all x, then the arc length of y = ƒ(x) over [a, b] is at most √2(b − a). Show that for ƒ(x) = x, the arc length equals √2(b − a).