a=(2x-y+1,x+y-2),b=(2,-2)
a.b
=(2m+n).(-3m+2n)
= -6|m|^2 +m.n + 2|n|^2
= -4+cos60°
= -7/2
also
a.b = |a||b|cosα ( α :a,b夾角)
-7/2= 2√ 2(√((2x-y+1)^2 + (x+y-2)^2)) cosα
cosα = -7√ 2/[8(√((2x-y+1)^2 + (x+y-2)^2))]