Let Tn denote the number of triangles which can be formed using the vertices of a regular polygon of 'n' sides. If Tn+1-Tn=21, then 'n' equals -

Dear student
The number of triangles that can be formed byusing the vertices of a regular polygon is nC3. That is Tn=nC3Now, Tn+1-Tn=21n+1C3-nC3=21nC2+nC3-nC3=21    n+1Cr=nCr-1+nCr12n(n-1)=21n=-6 or 7As n is a positive integer, n=7
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What do you mean Supurna Sadhukhan?? Can you please help answering the question??
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