∵tanθ=1/2
∴2=1/tanθ
sinθ*cosθ - 2(sinθ)^2
=sinθ*cosθ - (1/tanθ)*(sinθ)^2
=sinθ*cosθ -(cosθ/sinθ)*(sinθ)^2
=sinθ*cosθ -sinθ*cosθ
=0