a·b=(-2sinx,2cosx)·(-2√5,0)
=4√5sinx
|a|=2,|b|=2√5
故：cos=a·b/(|a|*|b|)=4√5sinx/(4√5)=sinx
=cos(π/2-x)
x∈(0,π/2),即：-x∈(-π/2,0)
即：π/2-x∈(0,π/2)
故：=π/2-x