= [n = 3 ][12] n- 1 C 2 = 2 C 2 + 3 C 2 +............+11 C 2
Now since n C r = n C n-r
So we have 3 C 0 + 3C1 + 4C2 +......+11C9
=4C1 + 4C2 +.....+ 11C9 = 12 C 9
Hence option B is correct.
Hope it helps.....Sorry for not being able to put the
vector signs.Nudge if not clear.Rate if satisfied.
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Not all who wander are lost
The old that is strong does not wither,
Deep roots are not reached by frost.
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From shadows a light shall spring
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The crown less again shall be king.
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[12] n- 1 C 2 = 2 C 2 + 3 C 2 +............+11 C 2 
