Question

Prove cos 2A + cos 2B + cos 2C + cos 2D = 4[cosAcosBcosCcosD - sinAsinBsinCsinD]
If A+B+C+D=0
@ikram002p @ganeshie8 @Hero

Answer 1

@mayankdevnani

Answer 2

Cos [ A + B +C +D] = Cos (0) = 1

Answer 3

Cos [(A + B) + (C+D)] = Cos (A+B)Cos(C+D) - Sin(A+B)Sin(C+D)
[Cos A Cos B - Sin A Sin B][Cos C Cos D - Sin C Sin D] - [Sin A Cos B + Cos A Sin B][Sin C Cos D + Cos C Sin D]

It might help. just needs to be simplified anyway.

Answer 4

not sure how that helps @ikram002p @ganeshie8 any ideas?

Answer 5

@lekshmyprasad ? any idea?