已知tanΘ=1/3,求cos2Θ+2sin2Θ

因为 tan Θ =1/3,所以 cos 2Θ +2 sin 2Θ = [ (cos Θ)^2 -(sin Θ)^2 +4 sin Θ cos Θ ] / [ (sin Θ)^2 +(cos Θ)^2 ]= [ 1 -(tan Θ)^2 +4 tan Θ ] / [ (tan Θ)^2 +1 ]= [ 1 -(1/3)^2 +4 *(1/3) ] / [ (1/3)...