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?
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How many triangles can I forme use all the vertices? ( 3 8 )
How many triangles can I forme use the vertices of one face? ( 3 4 )
Total is ( 3 8 ) − 6 × ( 3 4 ) = 3 2
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We have 8 vertices and we need to form a triangle with 3 vertices. Thus, there are exactly ( 3 8 ) different triangles.
On a face of a cube, there are 4 vertices. So, we can form ( 3 4 ) different triangles on a face. We have 6 faces, so there are 6 ∗ ( 3 4 ) triangles whose vertices lie on the same face. We subtract them from the number of all different triangles and we get our result:
( 3 8 ) − 6 ∗ ( 3 4 ) = 5 6 − 6 ∗ 4 = 3 2