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Question

If sin(xα)sin(xβ)=ab,cos(xα)cos(xβ)=AB and aB + bA 0, then prove that -
cos(αβ)=aA+bBaB+bA.

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Solution

Adding the given relation , we get
ab+AB=sin(xα+xβ)sin(xβ)cos(xβ)
aB+bAbB=sin(2xαβ)sin(xβ)cos(xβ)
Multiplying the given relation and adding 1 , we get
aAbB+1=sin(xα)cos(xα)+sin(xβ)cos(xβ)sin(xβ)cos(xβ)
or aA+bBbB
=12sin(2x2α)+sin(2x2β)sin(xβ)cos(xβ)
=122sin(2xαβ)cos(αβ)sin(xβ)cos(xβ)
Dividing (1) and (2) ,we get result

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