=tgt^2 + 2(tgt)(ctgt) + ctgt^2 - (tgt^2 - 2(tgt)(ctgt) + ctgt^2)
=tgt^2 + 2(tgt)(ctgt) + ctgt^2 - tgt^2 + 2(tgt)(ctgt) - ctgt^2
=4(tgt)(ctgt)
=4tgt^2