y=2-4/5sinx-(1-sin²x)
=2-4/5sinx-1+sin²x
=sin²x-4/5sinx+1
=(sinx-2/5)²+21/25
当sinx=2/5时,y值最小=21/25
当sinx=-1时,y值最大=（-1-2/5）²+21/25=49/25+21/25=14/5