This triangle, for the convenience of calculation, we divide into two **right-angled** triangles.

For example, a pair of equal opposite vortices, moving on a line parallel to a plane boundary, will have a corresponding pair of images, forming a rectangle of vortices, and the path of a vortex will be the Cotes' spiral r sin 20 = 2a, or x-2+y-2=a-2; (io) this is therefore the path of a single vortex in a **right-angled** corner; and generally.

To say, for instance, that the area of a **right-angled** triangle is half the area of the rectangle contained by the two sides, is not to say what the area is, but what it is the half of.

In E'; the case of a parallelogram, the equivalent right,, trapezium is a rectangle; in the case of a triangle, Al it is a **right-angled** triangle.

When hardened in situ its shape is that of a **right-angled**, triangular prism showing five surfaces - superior, anterior, inferior, posterior and right lateral which represents the base of the prism.