Question

Help please. Yusuf used the equation cos⁡ 80°=2 cos² 40°−1 to find cos⁡ 80°. Which trigonometric identity did Yusuf use?

sum identity for cosine

double-angle identity for cosine difference

identity for cosine

half-angle identity for cosine

Answer 1

Answer:

double-angle identity for cosine difference

Step-by-step explanation:

we know that

The Cosine of a Double Angle Formula is equal to

$cos(2A)=2cos^2(A) -1$

Let

$A=40^o$

substitute

$cos(2(40^o))=2cos^2(40^o) -1$

$cos(80^o)=2cos^2(40^o) -1$

therefore

Yusuf used the double-angle identity for cosine difference