Lengths of triangle sides using the Pythagorean Theorem to classify triangles as obtuse, acute or right. %

This is a free geometry lesson for 4th grade about acute, obtuse, and right triangles (classification according to angles). It contains varied exercises, including several where students explore these concepts -- and even the angle sum of a triangle -- by

An acute triangle is a triangle whose angles are all acute (i.e. less than 90°). In the acute triangle shown below, a, b and c are all acute angles. An equilateral triangle is always an acute triangle since all its angles are 60° which are acute angles.