# Definition:Right Circular Cone/Right-Angled

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## Definition

Let $K$ be a right circular cone.

Then $K$ is **right-angled** if and only if the opening angle of $K$ is a right angle.

In the words of Euclid:

*When, one side of those about the right angle in a right-angled triangle remaining fixed, the triangle is carried round and restored again to the same position from which it began to be moved, the figure so comprehended is a***cone**.

And, if the straight line which remains fixed be equal to the remaining side about the right angle which is carried round, the cone will be**right-angled**; if less,**obtuse-angled**; and if greater,**acute-angled**.