若tan a=1/3,则sin^2 a- sina*cosa -cos^2 a=
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若tan a=1/3,则sin^2 a- sina*cosa -cos^2 a=
![若tan a=1/3,则sin^2 a- sina*cosa -cos^2 a=](/uploads/image/z/16510418-26-8.jpg?t=%E8%8B%A5tan+a%3D1%2F3%2C%E5%88%99sin%5E2+a-+sina%2Acosa+-cos%5E2+a%3D)
(sina)^2-sina*cosa-(cosa)^2
=[(sina)^2-sina*cosa-(cosa)^2] / [(sina)^2+(cosa)^2] (添上分母 1)
=[(tana)^2-tana-1] / [(tana)^2+1] (分子分母同除以 (cosa)^2)
=(1/9-1/3-1) / (1/9+1)
= -11/10 .
=[(sina)^2-sina*cosa-(cosa)^2] / [(sina)^2+(cosa)^2] (添上分母 1)
=[(tana)^2-tana-1] / [(tana)^2+1] (分子分母同除以 (cosa)^2)
=(1/9-1/3-1) / (1/9+1)
= -11/10 .
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