Question

If cos ${20}^{\circ }=k$ and cos $x=2k^2-1$, then the possible value of x between $0^{\circ }$ and ${360}^{\circ }$ are

• A. ${140}^{\circ }$
  
• B. ${40}^{\circ }$ and ${140}^{\circ }$
  
• C. ${50}^{\circ }$ and ${130}^{\circ }$
  
• D. ${40}^{\circ }$ and ${320}^{\circ }$

Answer 1

The correct option is D ${40}^{\circ }$ and ${320}^{\circ }$

$cosx=2co{s}^2{20}^{\circ }-1=cos{40}^{\circ }=cos{320}^{\circ }$