已知sin(a-30)=8/17,30
问题描述:
已知sin(a-30)=8/17,30
答
300
==> cos(a-30)=根号下(1 - sin(a-30)^2) = 根号下(1 - (8/17)^2) = 15/17
==> cosa = cos(a-30 + 30) = cos(a-30)cos30 - sin(a-30)sin30 = (15倍根号3 - 8)/34
==> cosa = (15倍根号3 - 8)/34