cotθ=12/5=>tanθ=1/cotθ=5/121.
If cosθ=1/tanθ, then cosθ=1/tanθ=12/5.
There is no solution, because −1≤cosθ≤1 for θ∈R.
2.
Suppose that the condition cosθ=1/tanθ is False.
Then
1+cot2θ=sin2θ1
sin2θ=1+cot2θ1=1+(12/5)21=16925
cos2θ=1−sin2θ=1−16925=169144
cosθ+sinθcos(2θ)=cosθ+sinθcos2θ−sin2θ
=cosθ+sinθ(cosθ−sinθ)(cosθ+sinθ)
=cosθ−sinθ,cosθ=−sinθ i)
sinθ>0,cosθ>0 Then
sinθ=16925=135
cosθ=169144=1312
cosθ+sinθcos(2θ)=cosθ−sinθ
=1312−135=138
ii)
sinθ<0,cosθ<0 Then
sinθ=−16925=−135
cosθ=−169144=−1312
cosθ+sinθcos(2θ)=cosθ−sinθ
=−1312−(−135)=−138
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