Respuesta :
Since, tigonometric identities are equal on both the sides (left hand side and right hand side) of the equation, Options (B), (C), and (D) represent the dientities.
Identification of the tigonometric identites:
- Trigonometric identities are the equalities having two trigonometric expressions equal for all values of the variables on both the sides.
- To prove the expression as identity prove both the sides of the equation are equal.
Option A
cos(x + y) + cos(x - y) = cos²2x - sin²2y
Left hand side = cos(x + y) + cos(x - y)
= cosx.cosy - sinx. siny + cosx.cosy + sinx.siny
= 2(cosx.cosy)
But the left side of the equation is (cos²2x - sin²2y).
Hence both the sides are not equal and it's not an identity.
OPtion (B).
sin(x - π) = -sinx
Left hand side = sin(x - π)
= -sinx sin(x - π)
= sinx.cosπ - cosx.sinπ
= -sinx - 0
= -sinx [Right hand side]
Since, both the sides are equal, it's an identity.
Option (C).
sin(x + y) - sin(x - y) = 2cosx.siny
Left hand side = sin(x + y) - sin(x - y)
= sinx.cosy + cosx.siny - sinx.cosy + cosx.siny
= 2cosx.siny
= Right hand side
Since, both the sides are equal, it's an identity.
Option (D).
cos(x + y) + cos(x - y) = 2cosx.cosy
Left hand side = cos(x + y) + cos(x - y)
= cosx.cosy - sinx.siny + cosx.cosy + sinx.siny = 2cosx.cosy
= Right hand side
Since, both the sides of the equation are are equal. It's an identity.
Therefore, Option (B), (C), (D) are the correct options.
Learn more about the trigonometric identities here,
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