Question 18If $b\gt1,$ $\sin x\gt0,$ $\cos x\gt0$ and $\log_b\sin x=a,$ then $\log_b\cos x$ equals$2\log_b(1-b^{\frac{a}{2}})$$\sqrt{1-a^2}$$b^{a^2}$$\frac12\log_b(1-b^{2a})$none of these