Right Triangle

Definition: A right triangle is a triangle with exactly one 90 degree (right) angle. A right angle is indicated with a “box” or “square” symbol at the angle’s vertex.

A right triangle can also be isosceles if the two sides adjacent to the right angle are congruent.


Example: Find the measure of ∠x:


Solution: Use the information provided in the figure to solve for ∠x.

Step 1: We can determine from the figure’s markings that this is a right triangle with one 20 degree angle. Keeping the Triangle Sum Theorem in mind (the sum of a triangle’s interior angles equals 180 degrees), we can write the following equation to find x:
90 + 20 + x = 180

Step 2: Solve for x
110 + x = 180
x = 70°, so the right triangle’s missing angle is equal to 70°.

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