第991题：口诀
sin15°=?\sin 15 \degree = ?sin15°=?
A. 6−24\dfrac{\sqrt{6}-\sqrt{2}}{4}4√6−√2
B.6+24 \dfrac{\sqrt{6}+\sqrt{2}}{4}4√6+√2
C. 5−24\dfrac{\sqrt{5}-\sqrt{2}}{4}4√5−√2
D. 5+24\dfrac{\sqrt{5}+\sqrt{2}}{4}4√5+√2
sin(α−β)=\sin(\alpha - \beta)=sin(α−β)=sinαcosβ−cosαsinβ\sin \alpha \cos \beta - \cos \alpha \sin \betasinαcosβ−cosαsinβ
口决：正余余正，符号相同.
