Find cos15 if cos(a-b)=cosa cosb + sina sinb taking suitable a and b
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Given , cos (A-B) = cos A cos B + sin A sin B
cos 15 = cos (45-30)
== cos 15 = cos 45 cos 30 +sin 45 sin 30
== cos 15 = 1/ root 2* root3 /2 + 1/root2 * 1/2
==cos 15 = root 3/ 2 root2 + 1/ 2 root2
== cos 15 = (root3 +1)/ 2 root2
cos 15 = cos (45-30)
== cos 15 = cos 45 cos 30 +sin 45 sin 30
== cos 15 = 1/ root 2* root3 /2 + 1/root2 * 1/2
==cos 15 = root 3/ 2 root2 + 1/ 2 root2
== cos 15 = (root3 +1)/ 2 root2
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hope this may help you
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