Question

"Question28
Using the formula, cos A = √1+cos2A /2 , find the value of cos 30°, it being given that cos 60° = 1/2
Chapter6,T-Ratios of particular angles Exercise -6 ,Page number 289"

Answer 1

Hey there !

Given Formula : cosA = √1 + cos2A / 2
Also, cos60 = 1/2
Required to find : cos30

Now,
Let A = 30 , 2A = 60°

cosA = √1 + cos2A / 2

cos30 = √( 1 + 1/2 ) / 2

cos30 = √(3/2)/2

cos30 = √(3/4) = √3 / 2

Therefore , cos30 = $\frac {\sqrt 3} { 2 }$

Answer 2

Heya,

Acc. to the question!! ⬇

Put A = 30°

2A = 60°

Cos A = √1 + cos2A / 2

Cos30° = 1 + cos60° / 2

= (1+1/2) / 2 = √3 / 2 /2

= √3 / √2 × 1 / √2 = √3 / 2

So, cos 30° = √3 / 2

Thanks!!