sin^2(α+π)cos(π+α)cot(-α+2π)/tan(π-α)cos^3(-α-π)
=(-sinα)^2*(-cosα)*cot(-α)/(-tanα)cos^3(-α+π)
=(sinα)^2*cosα*cotα/tanαcos^3(-α)
=sinα*cosα*cosα/tanα(cosα)^3
=sinα*(cosα)^2/sinα(cosα)^2
=1