y=cosa^2+cosa*sina
=(cos2a+1)/2+sin2a/2
=(sin2a+cos2a)/2+1/2
=[根号2*sin(2a+45°)+1]/2
由于-1≤sin(2a+45°)≤1,
所以y的值域是[（-根号2+1）/2,（根号2+1）/2]