If for 3 r 30, (30C30-r )+3 (30C31-r )+3 (30C32-r )+ (30C33-r )=mCr, then m equals :
- 31
- 32
- 33
- 34
Answer
(3) 33
Solution
Put k=30-r. LHS =30Ck+3 30Ck+1+3 30Ck+2+30Ck+3
Using nCj+nCj+1=n+1Cj+1:
LHS = (30Ck+30Ck+1 )+2 (30Ck+1+30Ck+2 )+ (30Ck+2+30Ck+3 )
=31Ck+1+2 31Ck+2+31Ck+3
= (31Ck+1+31Ck+2 )+ (31Ck+2+31Ck+3 )=32Ck+2+32Ck+3
=33Ck+3=33C33-r=33Cr
m=33
Answer: (3)