Cho coss alpha = 3/4; sin alpha > 0. Tính cos 2alpha, sin alpha. sin alpha = căn 7/4

A. $\sin \alpha =\frac{\sqrt{7}}{4};cos2\alpha =\frac{1}{8}$

B. $\sin \alpha =-\frac{\sqrt{7}}{4};cos2\alpha =\frac{1}{8}$

C. $\sin \alpha =\frac{\sqrt{5}}{4};cos2\alpha =\frac{\sqrt{11}}{4}$

D. $\sin \alpha =-\frac{\sqrt{7}}{4};cos2\alpha =-\frac{1}{8}$