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`        Sec(A+B) + Sec(A-B) = 2SecA then prove that cosa = +-2cosa/2`
2 years ago

```							Dear Student 1/cos (A+B)  +  1/ cos (A-B) = 2 / cos Acos (A+B) + cos (A-B) / (cos (A+B)cos(A –B))   = 2/ Cos A 2 cos A cos B = (cos (A+B)cos(A –B)) / cosAcos2A cos B = (cos (A+B)cos(A –B)) cos2A cos B = cos2A – sin2Bcos2A (cos B – 1) = – sin2B cos2A (cos B – 1) = (cos B – 1) (cos B +1) cos2A  = (cos B +1) cos2A  = 2 cos2(B/2) cosA = sqrt * cos (B/2)  RegardsArun (askIITians forum expert)
```
2 years ago
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