Answer:
3/5
Step-by-step explanation:
We need to use the trig identity that cos(2A) = cos²A - sin²A, where A is an angle. In this case, A is ∠ABC. Essentially, we want to find cos∠ABC and sin∠ABC to solve this problem.
Cosine is adjacent ÷ hypotenuse. Here, the adjacent side of ∠ABC is side BC, which is 4 units. The hypotenuse is 2√5. So, cos∠ABC = 4/2√5 = 2/√5.
Sine is opposite ÷ hypotenuse. Here, the opposite side of ∠ABC is side AC, which is 2 units. The hypotenuse is still 2√5. So sin∠ABC = 2/2√5 = 1/√5.
Now, cos²∠ABC = (cos∠ABC)² = (2/√5)² = 4/5.
sin²∠ABC = (sin∠ABC)² = (1/√5)² = 1/5
Then cos(2∠ABC) = 4/5 - 1/5 = 3/5.
