If 20 C r -1-20 C r -1- then r is equal toA- 11B- 12C- 19D- 10

The correct option is D 10 r + 1 + r 1 = 20 [∵^nC_x = ^nC_y⇒ n = x + y or x = y] ⇒ 2r = 20 ⇒ r = 10 ...

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nCr Definitions and Properties

If 20 C r +1=...

Question

If $^{20} C_{r + 1} =^{20} C_{r - 1},$ then r is equal to

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