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The Question:

If someone drew a triangle with a 45 degree angle, could it be acute, right, or obtuse?

—whitney, December 05, 2001

Your Answer:

Well, first of all remember an acute angle is one that is less that 90 degrees and an obtuse angle is one that is greater than 90 degrees. Also all three angles of a triangle must add up to 180 degrees and an angle that is equal to 90 degrees is a right angle.
So if you know that one angle is 45 degrees, then you know that the other two angles must add up to 180-45 or 135 degrees. There are several ways this can happen. This best way to answer your question is to try an exmaple of each of the types of angles.
Acute: meaning that all 3 angles are less than 90 degrees. Well if you divide 135 by two you get two angles less than 90 degrees.
Obtuse: meaning that 1 angle is great than 90 degrees. Well if one angle is 100 degrees than the other could be 35 degrees.
Right: meaning that 1 angle is a right angle or 90 degrees. If that is true then your other angle would be 45 and you have an isosocles right triangle.
This page from Dr. Math might also help you to understand the types of angles in triangles.

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