Right triangle

A right triangle has one interior angle equal to a right angle.

A leg of a right triangle is either of the two sides meeting at the right angle. The opposite side is the hypotenuse. For leg lengths $a,b$ and hypotenuse $c$, the Pythagorean theorem gives $a^2+b^2=c^2$, while the area is $ab/2$. In the $3$-$4$-$5$ right triangle, the legs have lengths three and four. Relative to an acute angle, one leg is adjacent and the other opposite; these names depend on the chosen angle.

The hypotenuse is the side opposite the right angle in a right triangle. Its length $c$ is determined by the Pythagorean theorem.