已知tanθ=2. (Ⅰ)求tan(π4+θ)的值; (Ⅱ)求cos2θ的值.

问题描述:

已知tanθ=2.
(Ⅰ)求tan(

π
4
+θ)的值;
(Ⅱ)求cos2θ的值.

(Ⅰ)∵tanθ=2,
∴tan(

π
4
+θ)=
1+tanθ
1−tanθ
=
1+2
1−2
=-3;
(Ⅱ)∵tanθ=2,∴
sinθ
cosθ
=2,即sinθ=2cosθ,
∴sin2θ=4cos2θ,
∴1-cos2θ=4cos2θ,即cos2θ=
1
5

∴cos2θ=2cos2θ-1=-
3
5