[tex]$\sin 2\pi =0, \ \ \mathrm{tg}0=0, \ \ \cos \pi =-1, \ \ \sin (-\pi) =0$[/tex]

[tex]a^2\sin 2\pi+b^2\mathrm{tg}0+2ab\cos \pi+b^2\sin(-\pi)\\0+0-2ab+0=-2ab[/tex]