Question

find the value of cos18

Answer 1

Hey mate

your answer is here


Let A= 18°

5A=90°
2A=90°-3A
Sin 2A = Sin (90°-3A)= Cos3A

2SinA CosA=
$4cos {}^{3} a - 3cosa$

2SinA=
$4cos {}^{2} - 3 = 1 - 4sin {}^{2}$
$4sin {}^{2} + 2sin - 1 = 0$

$sin = ( \sqrt{5 - 1) \div 4}$
$sin {}^{2} = (1 - 2sin) \div 4 = (3 - \sqrt{5) \div 8}$
$cos = \frac{ \sqrt{5 + \sqrt{5} } }{8} = \frac{1}{4} \sqrt{10 + 2 \sqrt{5} }$
hope it helps

please mark it as brainliest friend