Types of Triangles

Classified by sides: equilateral (all equal), isosceles (two equal), scalene (all different). By angles: acute (all under 90°), right (one 90°), obtuse (one over 90°). These categories combine — a right isosceles triangle has a 90° angle and two equal legs.

The area formula is base × height ÷ 2, regardless of triangle type. Why divided by 2? Take any triangle, copy it, and rotate the copy 180° — the two pieces fit together into a parallelogram with the same base and height. Since that parallelogram's area is base × height, a single triangle is exactly half of it — and that's where the ÷ 2 comes from.