# altitude

## 01 Minimum distance between projection points on the legs of right triangle

**Problem**

From the right triangle ABC shown below, AB = 40 cm and BC = 30 cm. Points E and F are projections of point D from hypotenuse AC to the perpendicular legs AB and BC, respectively. How far is D from AB so that length EF is minimal?

## The Regular Tetrahedron

Regular tetrahedron is one of the regular polyhedrons. It is a triangular pyramid whose faces are all equilateral triangles.

## Regular Pyramid

A regular pyramid is one whose base is a regular polygon whose center coincides with the foot of the perpendicular dropped from the vertex to the base.

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## The Right Circular Cylinder

A right circular cylinder is a cylinder whose base is a circle and whose elements are perpendicular to its base.

## Properties of Triangle

**Side**

Side of a triangle is a line segment that connects two vertices. Triangle has three sides, it is denoted by *a*, *b*, and *c* in the figure below.

**Vertex**

Vertex is the point of intersection of two sides of triangle. The three vertices of the triangle are denoted by *A*, *B*, and *C* in the figure below. Notice that the opposite of vertex *A* is side *a*, opposite to vertex *B* is side *B*, and opposite to vertex *C* is side *c*.