Question

已知tanα=-2,则(sin^2 α-3cos^2 α)/( cos^2 α- sin^2 α)=?

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Answer 1

解

(sin²a-3cos²a)/(cos²a-sin²a)——分子分母同除以cos²a
=(tan²a-3)/(1-tan²a)
=(4-3)/(1-4)
=-1/3

Answer 2

tana=-2, ,
(sin^2 α-3cos^2 α)/( cos^2 α- sin^2 α)
=(tan^2 α-3)/( 1- tan^2 α)
=(4-3)/(1-4)
=-1/3