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Let Tn denote the number of triangles...

Let `T_n` denote the number of triangles, which can be formed using the vertices of a regular polygon of `n` sides. It `T_(n+1)-T-n=21 ,t h e nn` equals a.`5` b. `7` c. `6` d. `4`

A

5

B

7

C

6

D

4

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