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If cosA=4/5 and cos B=12/13 where and (3...

If `cosA=4/5` and `cos B=12/13` where and `(3pi)/2 < B < A < 2pi` then find `cos(A+B)`

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Givn,A and B both lie in the IV quadrant.
=> sinA,sinB are negative
`cosA=4/5​=>sinA=(−3)/5​`
`cosB=12/13​=>sinB=(−5)/13`
​ (i) cos(A+B)=cosA.cosB−sinA.sinB
...
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