Isosceles anglesThe two base angles are equal
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Find the angle marked x. The ticks mark the two equal sides.
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Angles in an isosceles triangle worksheets
An isosceles triangle has two equal sides, and that forces its two base angles to be equal. These free printable worksheets tick the equal sides, give one angle, and ask for the missing one — the apex from a base angle, or a base angle from the apex.
Kid stuck on one? You don’t just get the sheet.
Angles in an isosceles triangle
The answer key prints on a separate page.
What's on this sheet
The two facts do all the work: the base angles are equal, and the three angles add to 180°. From a base angle, double it and take the result off 180 to get the apex. From the apex, take it off 180 and halve what is left — the two base angles share it equally. Which way to go depends only on which angle is marked.
Why this sheet works
The ticks on the two equal sides are the whole reason the base angles are equal, so they are drawn rather than described — a triangle with no ticks is just a triangle, and the equal-angle fact would have nothing to stand on.
How this one works
One question from this sheet, worked through a step at a time.
Isosceles triangle angles
base=baseandsum=180°
180−130=50
x=50°
- The two ticked sides are equal, so the two base angles are equal — and the three angles still add to 180°.
- Both base angles are 65°, so together they are 130°. The apex is what is left of 180°: 180 − 130 = 50.
- So x = 50°.