π/2＜a＜π，sina=4/5所以cosa=-√[1-(4/5)^2]=-3/5所以cosa=2[cos(a/2)]^2-1又π/4＜a/2＜π/2所以cos(a/2)=√[(1+cosa)/2]=√[(1-3/5)/2]=√5/5如果不懂，请Hi我，祝学习愉快！
