c)
VT=sin^6a+cos^6a=(sin^2a)^3+(cos^2a)^3
=(sin^2a+cos^2a)(sin^4a+cos^4a-sin^2a.cos^2a)
=(sin^2a+cos^2a)^2-3.sin^2a.cos^2a
=1-3.sin^2a.cos^2a
=1-\frac{3}{4}.sin^22a
VP=\frac{5}{8}+\frac{3}{8}.cos4a
=\frac{5}{8}+\frac{3}{8}.(1-2sin^22a)
=\frac{5}{8}+\frac{3}{8}-\frac{3.2sin^22a}{8}
=1-\frac{3}{4}.sin^22a
=> đpcm