sinα-cosα=1/3(sinα-cosα)^2=1/9(sinα)^2+(cosα)^2-2sinαcosα=1/91-2sinαcosα=1/92sinαcosα=8/9sin2α=8/9cos2α=±√17/9cos4α=(cos2α)^2-(sin2α)^2=17/81-64/81=-47/81