Question

Prove that: `s in" "x" "+" "s in" "3x" "+" "s in" "5x" "+" "s in" "7x" "=" "4" "cos" "x" "cos" "2x" "s in" "4x`

Answer 1

LHS `=sinx+sin3x+sin5x+sin7x = (sin7x+sin5x)+(sin5x+sin3x)`
`=2sin(7x+x)/(2)cos(7x-x)/(2) + 2sin(5x+3x)/(2)cos(5x-3x)/(2)`
`=2sin4xcos3x+2sin4xcosx`
`=2sin4x(cos3x+cosx)`
`=2sin4x[2cos(3x+x)(2)cos(3x-x)/(2)]`
`=2sin4x[2cos2xcosx]`
`=4cosxcos2xsin4x`= RHS Hence Proved.