Understanding that angles in a triangle always add up to 180° and using this to find missing angles.
Where your child meets this in real life: Calculating angles in roof trusses, triangular structures, or navigation
SEAGReady breaks angles in triangles into 3 steps, taught in order so each skill builds on the last.
Find the missing third angle when two angles of a triangle are given
Find angles in isosceles and equilateral triangles using equal-angle properties
Solve multi-step problems combining triangle angles with other angle rules
Three free sample questions from our angles in triangles course. Every question comes with a full explanation, and hints that guide without giving the answer away.
Aoife is making a triangular pennant for her school. Two of the angles measure 60 degrees and 65 degrees. What is the third angle?
Answer: A. 55 degrees
Angles in a triangle always add to 180 degrees. Step 1: Add the known angles: 60 + 65 = 125 degrees Step 2: Subtract from 180: 180 - 125 = 55 degrees The third angle is 55 degrees.
Stuck? Start here: What do all the angles in a triangle add up to?
Emma is making an isosceles triangle for an art project. The top angle (apex) is 50 degrees. What is the size of each base angle?
Answer: A. 65 degrees
In an isosceles triangle, the two base angles are equal. Step 1: Subtract the apex angle from 180: 180 - 50 = 130 degrees Step 2: Divide by 2 for each base angle: 130 / 2 = 65 degrees Each base angle is 65 degrees.
Stuck? Start here: In an isosceles triangle, the two base angles are equal.
In triangle PQR, angle P is 45 degrees and angle Q is 70 degrees. Side QR is extended to point S. What is the exterior angle PRS?
Answer: A. 115 degrees
Step 1: Find interior angle R. 45 + 70 = 115 degrees 180 - 115 = 65 degrees Step 2: Find exterior angle PRS. Interior and exterior angles are on a straight line. 180 - 65 = 115 degrees The exterior angle PRS is 115 degrees.
Stuck? Start here: First find interior angle R using the triangle angle sum (180 degrees).
This is the exact interactive worked example your child sees in SEAGReady. Step through it and watch the method build up.
Ciaran is designing a triangular flag for his school's sports day. Two of the angles measure 55 degrees and 70 degrees.
What is the third angle of the flag?
55° + 70° + ? = 180°
Step 1 of 4
Ciaran is designing a triangular flag for his school's sports day. Two of the angles measure 55 degrees and 70 degrees.
What is the third angle of the flag?
The third angle of the flag is 55°.
The key insight: Every triangle's angles add to exactly 180 degrees - no matter what shape it is!
Watch out: Using 360° instead of 180°. 360° is for angles around a point. Triangles always use 180°.
These are the misconceptions we see most often in angles in triangles, including the ones our practice questions are specifically designed to catch.
Struggling with angles in triangles? The real gap is often in one of these earlier topics.
SEAGReady finds the exact step where your child gets stuck, teaches it with worked examples like the one above, and brings it back for review so it sticks.