Triangles In a Cube !!!

Any three vertices of a cube form a triangle. What is the number of all such triangles whose vertices are not all on the same face of the cube?


The answer is 32.

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3 solutions

Patrick Engelmann
Sep 29, 2014

We have 8 vertices and we need to form a triangle with 3 vertices. Thus, there are exactly ( 8 3 ) \binom{8}{3} different triangles.
On a face of a cube, there are 4 vertices. So, we can form ( 4 3 ) \binom{4}{3} different triangles on a face. We have 6 faces, so there are 6 ( 4 3 ) 6 * \binom{4}{3} triangles whose vertices lie on the same face. We subtract them from the number of all different triangles and we get our result:

( 8 3 ) 6 ( 4 3 ) = 56 6 4 = 32 \binom{8}{3} - 6 * \binom{4}{3} = 56 - 6 * 4 = \boxed{32}

Paola Ramírez
Jan 26, 2015

How many triangles can I forme use all the vertices? ( 8 3 ) \binom{8}{3}

How many triangles can I forme use the vertices of one face? ( 4 3 ) \binom{4}{3}

Total is ( 8 3 ) 6 × ( 4 3 ) = 32 \binom{8}{3}-6\times\binom{4}{3}=\boxed{32}

Fox To-ong
Feb 17, 2015

Thats 56 - 6x4 = 32

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