Cau 10:
[tex]\iff 4cosx^3 +6\sqrt{2}sinxcosx=8cosx \iff cosx(4cosx^2+6\sqrt{2}sinx-8)=0 \iff cosx(2(1-2sinx^2)+3\sqrt{2}sinx-4)=0[/tex]
Cau 13:
$\iff 4sinxcosx(cosx^4-sinx^4)=4sinxcosx(cosx^2+xinx^2)(cosx^2-sinx^2)=2sin2xcos2x=sin4x=sin4x^2$
$\iff sin4x(1-sin4x)=0$