cos^2(3pi/5) +cos^2(4pi/5)=?
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Thank you for asking this question. Here is your answer:
cos (A+B) = cos A cos B - sin A sin B
cos A cos B - sin A sin B = cos ( A+B)
cos (2π/5) cos (3π/5) - sin (2π/5) sin (3π/5)
= cos (2π/5 + 3π/5)
= cos (5π/5)
cos (2π/5) cos (3π/5) - sin(2π/5) sin (3π/5) - 1
If there is any confusion please leave a comment below.
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