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Question
Find the value of `cos-1 [cos pi/7 cos pi/17 - sin pi/7 sin pi/17]`
Solution
`cos-1 [cos pi/7 cos pi/17 - sin pi/7 sin pi/17]`
= `cos^1[cos(pi/7 + pi/17)]`
∵ cos(A + B) = cos A cos B – sinA sin B
= `cos1 [cos((17pi + 7pi)/119)]`
= `cos^1[cos((24pi)/119)]`
= `(24pi)/119`
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