cosa=1/5,a属于(3兀/2,2兀 ),sin(a+兀/3)=?

问题描述:

cosa=1/5,a属于(3兀/2,2兀 ),sin(a+兀/3)=?

因为cosa=1/5 ,且a属[3/2兀,2兀]
所以sina=-2√ 6/5
所以 sin(a+兀/3)=sinacos(兀/3)+cosasin(兀/3)
=(-2√ 6/5)*(1/2)+(1/5)*(√ 3/2)
=(√ 3-2√ 6)/10

sina=-2√6/5
sin(a+π/3)=1/2*sina+√3/2*cosa
=-1/2*2√6/5+√3/2*1/5
=(√3-2√6)/10

a第四象限
sina