CBSE Class 11-science Answered
Sir please tell me some easy method to prove the following:-
Asked by ashish sharma | 23 May, 2012, 02:37: PM
Expert Answer
cos (x + y)
=sin (90 - (x + y))
=sin (90 - x - y)
=sin (90 - x) cos (-y) + sin (-y) cos (90 - x)
=cos x cos y - sin x sin y ...(i)
substituting y by -y in (i)
cos (x-y) = cos x cos (-y) - sin x sin (-y)
= cos x cos y + sin x sin y {since cos(-x)=cos x and sin (-x) = -sin x}
sin (x + y)
=cos (90 - (x + y))
=cos ((90 - x ) - y)
=cos (90 - x) cos y + sin y sin (90 - x)
=sin x cos y + cos x sin y ...(i)
substituting y by -y in (i)
sin (x-y) = sin x cos (-y) + cos x sin (-y)
= sin x cos y - cos x sin y {since cos(-x)=cos x and sin (-x) = -sin x}
Answered by | 24 May, 2012, 11:20: AM
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