Question

cos^2(3pi/5) +cos^2(4pi/5)=?

Answer 1

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

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