Question 6·Hard·Right Triangles and Trigonometry
In the figure above, and the length of is 60.
Which choice is the length of ?
When a diagram shows a right triangle with an altitude drawn from the right angle to the hypotenuse, look for two moves: (1) use the given trig ratio to recognize a special right-triangle ratio (like --) in one of the smaller triangles to find missing side lengths, and (2) use similarity/altitude relationships such as to connect the small triangle to the entire triangle.
Hints
Identify the right triangle that uses the given sine
The sine expression involves , so focus on triangle and use the right-angle marking at .
Use a known right-triangle ratio
If , then the sides of triangle follow a -- pattern (scaled by some factor). Use to find that factor.
Connect triangle to triangle
Because is drawn to the hypotenuse, triangle is similar to the large right triangle. Use the relationship .
Desmos Guide
Compute the scale factor
In Desmos, type k=60/12 to compute the scale factor .
Compute and from the -- ratio
Type a_d=5k and a_c=13k (Desmos will treat these as variables and ).
Use the altitude relationship to compute
Type a_b=a_c^2/a_d and read the value of a_b.
Step-by-step Explanation
Use the sine ratio in right triangle
From the diagram, is a right angle, so triangle is a right triangle.
For ,
So triangle matches the -- ratio:
Use to find the scale factor
Since and ,
Then
Relate a leg to the hypotenuse using the altitude
From the diagram, triangle is a right triangle with an altitude from to the hypotenuse at point . In this situation, triangle is similar to triangle , which leads to the geometric-mean relationship
Solve for
Substitute and into :
Therefore, the length of is 169.