If one angle of a triangle measures what is the measure of the third angle?

The sum of the three angles of a triangle is 180 degrees

If it is a right angle triangle one angle must be 90 degrees

All u have to do is 180-90- (the angle u already know)= unknown angle (third angle)

Just like regular numbers, angles can be added to obtain a sum. Sometimes we can determine a missing angle because we know that the sum must be a certain value. Remember -- the sum of the degree measures of angles in any triangle equals 180 degrees. Below is a picture of triangle ABC, where angle A = 60 degrees, angle B = 50 degrees and angle C = 70 degrees.

If we add all three angles in any triangle we get 180 degrees. So, the measure of angle A + angle B + angle C = 180 degrees. This is true for any triangle in the world of geometry. We can use this idea to find the measure of angle(s) where the degree measure is missing or not given.

Sample A

In triangle ABC below, angle A = 40 degrees and angle B = 60 degrees. What is the measure of angle C?

We know that the sum of the measures of any triangle is 180 degrees. Using the fact that angle A + angle B + angle C = 180 degrees, we can find the measure of angle C.angle A = 40

angle B = 60

angle C = we don't know.

To find angle C, we simply plug into the formula above and solve for C.A + B + C = 180

C = 180 - A - B

C = 180 - 40 - 60

C = 80

To check if 80 degrees is correct, let's add all three angle measures. If we get 180 degrees, then our answer for angle C is right.

Here we go:40 + 60 + 80 = 180

180 = 180...It checks!

You don't always have to plug in those values to the equation and solve. Once you're comfortable with this sort of problem you'll be able to say "okay, 40 + 60 =100, so the other angle has to be 80!" and it's much quicker.

Sample B

If a triangle is equiangular, what is the degree measure of each of its angles?

Remember, all angles of an equiangular triangle have equal measure. Let x = the degree measure of each angle. Triangles have three vertices and so we will add x THREE times.

We have this:x + x + x = 180

3x = 180

x = 60

Makes sense, right? If all the angles are equal, and they add up to 180, then it has to be 60 degrees!

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