Mathematics - Trigonometric Ratios & Identities Question with Solution | TestHub

A reaches 1 when $\cos x=0$. By Cauchy-Schwarz, B has maximum $\sqrt{6/5}\sqrt{1/2+1/3}=1$. For C, ${\cos }^{6}x+{\sin }^{6}x=$ $1-3{\sin }^{2}x{\cos }^{2}x\le 1$. D equals $1-{\sin }^{2}x{\cos }^{2}x\le 1$.