Answer:
First problem: x = √15
Second problem: x = 6√5 (B)
Third problem: x = 6, y = 12 (A)
Fourth problem: x = 6√2 (C)
Explanation:
We use geometric means as follows:
First problem:



Second problem:



So B is the correct answer.
Third problem:
We have a 30°-60°-90° triangle.
x = 6, y = 12. So A is the correct answer.
Fourth problem:
We have a right isosceles triangle.
x = 12/√2 = 6√2. So C is the correct answer.