Question

CosA =1/2
then .CoS 2 A =​

Answer 1

Step-by-step explanation:

METHOD (1) :-

If, CosA = 1/2 = Cos(π÷3)

=> A = (π÷3)

=> 2A = (2π÷3) = (π/2 + π/6)

then, Cos2A = Cos(π/2 + π/6)

= -Sin (π/6) = -1/2

OR

METHOD (2) :-

$Cos2A = 2Cos^{2}A - 1 \\ = 2 {(1 \div 2)}^{2} - 1 \\ = 2 \times (1 \div 4) - 1 \\ =(1 \div 2) - 1 = - 1 \div 2$

Answer 2

Answer:

∴cos 2A = $\frac{-1}{2}$

Step-by-step explanation:

cos A = $\sqrt{\frac{1+cos 2A}{2} }$

=>$(\frac{1}{2} )^2=\frac{1+cos 2A}{2}$

=>$\frac{2}{4} =1+cos 2A$

=>$\frac{1}{2}-1$ = cos 2A

=>cos 2A = -$\frac{1}{2}$