An isosceles triangle has only two equal sides.
A scalene triangle has all its sides unequal.
A rectangular, or right-angled triangle has one of its angles a right one, or 90°; and the square of the side opposite the right angle is equal to the sum of the squares of the sides containing that angle; hence a triangle, having its sides proportional to the numbers 3, 4, 5, will be right-angled.
The hypothenuse is the side opposite the right angle in a rectangular triangle.
An obtuse-angled triangle has one of its angles obtuse.
An acute-angled triangle has all its angles acute.
The three angles of any triangle, taken together, are equal to two right angles, or 180°.
The difference of the squares of two sides of a triangle is equal to the product of their sum and difference.
The sides of a triangle are proportional to the sines of their opposite angles.
Quadrangles, or quadrilaterals, are plane figures bounded by four right lines.