if cos (x-y) = a cos (x+y), then cotx coty is equal
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By compenendo and divedendo
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To find : cos(x+y) / cos(x-y)
==) (cos x cos y -sin x sin y)/ (cos x cos y + sin x sin y)
==) (2 sin x sin y - sin x sin y)/ (2 sin x sin y + sin x sin y) 【substitution】
==) sin x sin y / 3× sin x sin y
==) 1× sin x sin y/ 3× sin x sin y
==) 1/3 ans.
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