Question 27·Hard·Right Triangles and Trigonometry
In right triangle , angle is the right angle. If and the area of triangle is square units, what is the length of the hypotenuse ?
For right-triangle trig questions, first identify the hypotenuse (opposite the right angle) and label the sides relative to the given angle (opposite, adjacent, hypotenuse). Translate the trig ratio into a simple side ratio using a scale factor , then use the Pythagorean theorem to find the third side as another multiple of —often you will recognize a Pythagorean triple like ––. Finally, use any extra information given (such as area, perimeter, or another trig ratio) to solve for , and plug back in to get the requested side, being careful to answer exactly what the question asks (leg vs. hypotenuse).
Hints
Which side is the hypotenuse?
The hypotenuse is always the side opposite the right angle. Angle is , so which side is across from ?
Translate the sine ratio into side lengths
means . Decide which sides are “opposite” and “hypotenuse,” then write them as and for some scale factor .
Find the third side
Once you have two sides written as and , use the Pythagorean theorem to find the third side in terms of . You should get another multiple of .
Use the area to solve for the scale factor
The area of a right triangle is . Use your expressions for the legs in terms of , set this equal to , solve for , and then plug into your expression for the hypotenuse.
Desmos Guide
Confirm the third side using Pythagorean theorem
In Desmos, type sqrt(17^2 - 8^2) to verify that the third side in the right triangle is 15 (so the legs are 8 and 15 in some scale).
Set up the area expression in terms of k
Use the fact that the legs are and . In Desmos, enter y = 0.5*8*15*k^2 (which is y = 60k^2) as one expression and y = 180 as another.
Find the scale factor k
Look at the graph of and the horizontal line . Use the intersection tool or tap the point where they meet to read the value of that satisfies the area condition.
Compute the hypotenuse from k
Once you know , type 17*k into Desmos. The output is the length of the hypotenuse ; match this value to the answer choices.
Step-by-step Explanation
Identify the hypotenuse and the sides in the sine ratio
Angle is the right angle, so the hypotenuse is the side opposite angle , which is .
For angle :
- The opposite side is .
- The hypotenuse is .
By definition of sine,
So and must be in the ratio .
Express two sides using a scale factor
Let the common scale factor be .
Now use the Pythagorean theorem to find the third side :
so .
The side lengths are , , and .
Use the area formula for a right triangle to find the scale factor
In a right triangle, the area is
The legs are and , so
We are told the area is square units, so
Solve for :
Find the hypotenuse length
We want the length of the hypotenuse , which we wrote as . Substitute :
So the hypotenuse has length \17\sqrt{3}$, which matches choice B.