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JEE MATHEMATICS MCQ: COMBINATIONS

1 Easy

1 of 1

Application +4 | -1

Let

T_n

denote the number of triangles which can be formed using the vertices of a regular polygon of

n

sides. If

T_{n+1}-T_n=21

, then

n

equals

View answer and explanation

Correct option: A

7

Option A (7) is correct because the equation

{ }^{n+1} C_3-{ }^n C_3=21

simplifies to

{ }^n C_2=21

, which yields

n=7

.

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{\;^{n + 2}}{C_8}:{\;^{n - 2}}{P_4} = \frac{{57}}{{16}}

n