Answer:
y = 120°
Explanation:
Given a triangle with external angle y and remote internal angles 33° and 87°, you want the measure of y.
External angle
The external angle is the sum of the remote internal angles:
y = 33° +87°
y = 120°
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Additional comment
Angle x is supplementary to y, so its measure is ...
x = 180° -y = 180° -120°
x = 60°
Hopefully, you notice that the value of x is ...
x = 180° -(33° +87°)
This is the same value you get when you solve the equation for the sum of angles in the triangle:
33° +87° +x = 180°
Effectively, the rule regarding the external angle being the sum of remote internal angles is another way of saying the sum of angles in a triangle is 180°.