Answer:
c 15
Step-by-step explanation:
Tangent AB is tangent to circle D at point A. The angle made by a tangent to a circle and a radius of the circle at the point of tangency is a right angle. That means that angle is a right angle, and triangle ABD is a right triangle.
We can use the Pythagoren theorem.
a^2 + b^2 = c^2
x^2 + 20^2 = (x + 10)^2
x^2 + 400 = (x + 10)(x + 10)
x^2 + 400 = x^2 + 10x + 10x + 100
400 = 20x + 100
20x = 300
x = 15
