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Questions  

If cosA+cosB=m and sinA+sinB=n where m,n0, then sin(A+B) is equal to

a
mnm2+n2
b
2mnm2+n2
c
m2+n22mn
d
mnm+n

detailed solution

Correct option is B

We have, mn=(cos⁡A+cos⁡B)(sin⁡A+sin⁡B)⇒mn=cos⁡Asin⁡A+sin⁡(A+B)+sin⁡Bcos⁡B⇒2mn=sin⁡2A+sin⁡2B+sin⁡(A+B)⇒2mn=2sin⁡(A+B)cos⁡(A−B)+sin⁡(A+B)           ... (i)Also, we have m2+n2=2+2cos⁡(A−B)                 ... (ii)From (i) and (ii), we havesin⁡(A+B)=2mnm2+n2

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