Find the marked angles
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the angle closest to the 40degree angle is a right angle, so it is 90 degrees. The one with two lines is 50 degrees, and the farthest angle is 40 degrees

Step-by-step explanation:

A square symbol over an angle means it is a right angle, so it will always equal 90 degrees.

The already labeled angle (40 degrees) is equal to the farthest angle.

Once you have two angles, you can find the third angle by simply subracting the two from 180 like this:

180 - 90 - 40 = 50 degrees.