Q. 75.0( 1 Vote )

Find the value of cos18°

Answer :

FORMULA: - (i) in(90° – θ) = cosθ


(ii) sin2A = 2sinAcosA


(iii) cos3A = 4Cos3 A – 3 Cos A


(iv) sin2A + cos2A = 1


(v) for equation ax2 + bx + c



Let A = 18° then 5A = 90°


2A = 90° – 3A


Sin 2A = Sin (5A – 3A)


using formula (i)


sin2A = Sin (90° – 3A) = Cos 3A


using formula (ii) and (iii)


2 Sin A Cos A = 4Cos3 A – 3 Cos A


2 Sin A = 4 Cos2 A – 3


2sinA = 1 – 4 Sin2 A


using formula (iv)


4 Sin2 A + 2 Sin A – 1 = 0


using formula (v)



using formula (iv)




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