Angles, Lines, and Symmetry
Aligned to 4.G.A.1 — Common Core State Standards for Mathematics.
What this lesson teaches
Points, lines, rays, and angles are the building blocks of geometry. A right angle is a square corner (90 degrees), an acute angle is smaller, and an obtuse angle is larger. A shape has a line of symmetry if you can fold it so both halves match exactly.
Worked example
The corner of a piece of paper is a right angle, 90 degrees. An angle that opens less than that, like the tip of a slice of pizza, is acute.
Practice questions
- Is a 45 degree angle acute or obtuse?
- How many degrees is a right angle?
- Does a square have a line of symmetry? How many?
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
Ángulos, Rectas y Simetría
Los puntos, las rectas, los rayos y los ángulos son los elementos básicos de la geometría. Un ángulo recto forma una esquina perfecta (90 grados), un ángulo agudo es más pequeño y un ángulo obtuso es más grande. Una figura tiene un eje de simetría si puedes doblarla de manera que ambas mitades coincidan exactamente.
Ejemplo: La esquina de una hoja de papel es un ángulo recto de 90 grados. Un ángulo que se abre menos que eso, como la punta de un triángulo de pizza, es agudo.
More Geometry 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