Answer:
See work, below.
Explanation:
The radius of this circle is 10 and so the diameter is 20. The measure of the angle opposite side x is found as follows:
adj side
cos theta = ------------------- = 18/20. Find theta:
hypotenuse
theta = arccos 18/20 = 0.451 radian.
Now use the sine function to find x:
x x
sin 0.451 radian = ------------------- = ------------- = 0.651
hypotenuse 20
which results in x = 20(0.651) = 13.02
x = 13.02 units
I should have used the Pythagorean Theorem up front.
18² + x² = 20² becomes x² = 400 - 324 = 76, and so x = √76 ≠ 8.72