Question

cos-1x/2+cos-1y/2=a

prove that

9x^2-12xycosa+4y^2=36sin^2a

Answer 1

Explaination

cosider cos-1x/2=c

cos-1y/2=d

solve these for sin as well

now, c+d=a

take cos on both the sides and apply formula for cos(c+d)

upon solving you will get the required result

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