设a为锐角,若cos(a+π/6)=4/5,则sin(2a+π/12)的值为多少?
问题描述:
设a为锐角,若cos(a+π/6)=4/5,则sin(2a+π/12)的值为多少?
答
设b=a+π/6,sinb=3/5,sin2b=2sinbcosb=24/25,cos2b=7/25
sin(2a+π/12)=sin(2a+π/3-π/4)=sin(2b-π/4)=sin2bcosπ/4-cos2bsinπ/4=(17√2)/50