Question

cos^2(18°)+cos^2(36°)+cos^2(54°)+cos^2(72°)+cos^2(90°)​

Answer 1

Answer:

Step-by-step explanation:

Cos²36+cos²72 =>cos²36+cos²(90-18) ... 18)² - - - - - - (I) Now, sin 36 = cos 54 - - {compl. rule } ... 2 sin A = 4 cos² A - 3 - - - {Divide by cos A } => 2 sin A = 4 ... sin² 18 + ( 1 - 2sin² 18)² = (3 - √5)/8 + [1 - 2 (3 - √5)/8]² => (3 - √5)/8 .