Yusuf used the double-angle identity for cosine difference ⇒ 2nd answer
Step-by-step explanation:
Let us revise the identities of double an angle
- sin(2x) = 2 sin(x) cos(x)
- cos(2x) = cos²(x) - sin²(x)
- cos(2x) = 2 cos²(x) - 1
- cos(2x) = 1 - 2 sin²(x)
∵ Yusuf used the equation cos(80)° = 2 cos²(40)° - 1
∵ 80 = 2 × 40
∴ 80 is the double of 40
∵ cos(2x) = 2 cos²(x) - 1
∴ 2x = 80° and x = 40°
∴ The identity is double-angle identity for cosine difference
Yusuf used the double-angle identity for cosine difference
Learn more:
You can learn more about the trigonometry identities in brainly.com/question/2285189
#LearnwithBrainly