Question

Prove the identity (cosx+cosy)^2+(sinx-siny)^2= 2+2cos(x+y)

Answer 1

$TRIGONOMETRIC \\ \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: RESOLUTIONS \\ \\ \\\\Given \: expression \: - \\ \\ \\ { (\cos(x) + \cos(y)} )^{2} + ( {( \sin(x ) - \sin(y) )}^{2} \\ \\ = { \cos(x) }^{2} + { \cos(y) }^{2} + 2 \cos(x) \cos(y) \\ \: \: \:+\: \: { \sin(x) }^{2} + { \sin(y) }^{2} - 2 \sin(x) \sin(y) \\ \\ = { \cos(x) }^{2} \: + { \sin(x) }^{2} + \\ \: \: \: \: \: { \cos(y) }^{2} + { \sin(x) }^{2} \: + \\ \: \: \: \: \: 2( \cos(x) \cos(y) - \sin(x) \sin(y) ) \\ \\ = \: 1 + 1 + 2( \cos(x + y) ) \\ \\ Using \:Trigonometric \: Identities \: - \\ \\ { \sin(q) }^{2} + { \cos(q) }^{2} = \: 1 \: \\ \cos(x) \cos(y) - \sin(x) \sin(y) = \cos(x + y) \\ \\ \\ \\ { (\cos(x) + \cos(y)} )^{2} + ( {( \sin(x ) - \sin(y) )}^{2} \\ = 2 + 2( \cos(x + y) ) \: \: \: \: \: \: \: \: \: \: \: \: \: \: Ans.$

Answer 2

(cos(x) + cos(y))^2 + (sin(x) - sin(y))^2 Remove the brackets

cos^2(x) + cos^2(y) + 2cos(x)*cos(y) + sin^2(x) - 2(sin(x)*sin(y) + sin^2(y) Combine these two in bold to make 1 because sin^2(x) + cos^2(x) = 1

1 + cos^2(y) + 2cos(x)*cos(y) - 2*sin(x)*cos(y) + sin^2(y) 
These two in bold also make 1

2 + 2cos(x)*cos(y) - 2*sin(X)*sin(y) Bring out a common factor of 2
2 +2(cos(x)*cos(y) -  sin(x)*sin(y) )

but cos(x+y ) = cos(x)*cos(y) - sin(x)*sin(y)

2 + 2* cos(x + y) is your final answer.