Adding and Subtracting Angles
Aligned to 4.MD.C.7 — Common Core State Standards for Mathematics.
What this lesson teaches
When a big angle is split into smaller angles that do not overlap, the small angle measures add up to the big one. If you know the whole and one part, subtract to find the missing part.
Worked examples
Example 1
A 80 degree angle is split into two parts. One part is 30 degrees. What is the other part? The two parts add to 80, so subtract the known part: 80 - 30 = 50. The missing angle is 50 degrees. Check: 30 + 50 = 80.
Example 2
Two angles sit side by side on a straight line. One is 65 degrees. The other must be 180 − 65 = 115 degrees.
Example 3
A full turn is 360. If three angles around a point measure 120, 150, and ?, then ? = 360 − 120 − 150 = 90 degrees.
Practice questions
- Two angles of 25 and 40 degrees sit side by side. What is the total angle?
- A 90 degree angle is split into 55 degrees and one unknown angle. Find the unknown.
- An angle of 120 degrees is divided into three equal parts. How big is each part?
Watch the lesson
Every lesson comes with a video taught in English and Spanish — the same video the QR code in the printed workbook opens.
▶ Watch this lessonEn español
Sumar y restar ángulos
Cuando un ángulo grande se divide en ángulos más pequeños que no se traslapan, las medidas de los ángulos pequeños se suman para dar el grande. Si conoces el total y una parte, resta para hallar la parte que falta.
Ejemplo 1: Un ángulo de 80 grados se divide en dos partes. Una parte es de 30 grados. ¿Cuánto mide la otra parte? Las dos partes suman 80, así que resta la parte conocida: 80 - 30 = 50. El ángulo que falta mide 50 grados. Comprueba: 30 + 50 = 80.
Ejemplo 2: Dos ángulos están uno junto al otro sobre una línea recta. Uno mide 65 grados. El otro debe medir 180 − 65 = 115 grados.
Ejemplo 3: Una vuelta completa son 360. Si tres ángulos alrededor de un punto miden 120, 150 y ?, entonces ? = 360 − 120 − 150 = 90 grados.
More Measurement & Data lessons
Teach this lesson today
An AI tutor that guides with questions instead of just giving answers — all K–12 core subjects, every child in the family.
Start free — no card required