-1<=cosA<=1
-2<=-2cosA<=2
0<=2-2cosA<=4
所以1>=sinA>=0
sinA=2-2cosA
2cosA=2-sinA 平方
4cos^2 A=4-4sin^2A=4-4sinA+sin^2A
5sin^2A-4sinA=0
sinA(5sinA-4)=0
sinA=0或sinA=4/5