Prove \(\displaystyle{\sin{{8}}}\theta-{\sin{{10}}}\theta={\cot{{9}}}\theta{\left({\cos{{10}}}\theta-{\cos{{8}}}\theta\right)}\)

Anika Boyd

Anika Boyd

Answered question

2022-03-31

Prove sin8θsin10θ=cot9θ(cos10θcos8θ)

Answer & Explanation

Cason Singleton

Cason Singleton

Beginner2022-04-01Added 13 answers

From the angle addition identities
sin(a±b)=sinacosb±sinbcosa
cos(a±b)=cosacosbsinasinb
we find
sin(a+b)sin(ab)=2sinbcosa
cos(a+b)cos(ab)=2sinasinb
Then choosing a=9θ,b=θ, we get
sin10θsin8θ=2sinθcos9θ
cos10θcos8θ=2sin9θsinθ
Consequently,
sin8θsin10θcos10θcos8θ=2sinθcos9θ2sin9θsinθ=cot9θ

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