[tex]\displaystyle\int_0^{\pi/4}\sqrt{1+\sec^4x}\,\mathrm dx=\int_0^{\pi/4}\sqrt{1+(\sec^2x)^2}\,\mathrm dx[/tex]
Recall that [tex]\displaystyle(\tan x)'=\sec^2x[/tex]. Then right away you see the integral gives the arc length of the curve [tex]y=\tan x[/tex] over the given interval.