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Binomial Theorem question

2025 · 22 Jan · Shift 2 · Q48
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  5. /2025 · 22 Jan · Shift 2 · Q48

Binomial Theorem question

2025 · 22 Jan · Shift 2 · Q48

JEE MainMathematicsBinomial TheoremNumerical+4 / −1
If ∑r=130r2(30Cr)230Cr−1=α×229\sum_{r=1}^{30} \frac{r^2\left({ }^{30} C_r\right)^2}{{ }^{30} C_{r-1}}=\alpha \times 2^{29}∑r=130​30Cr−1​r2(30Cr​)2​=α×229, then α\alphaα is equal to ‾\underline{\hspace{2cm}}​.
Numerical answer
View written solutionFree

Correct answer: 465

  1. We need to evaluate S=∑r=130r2(30r)2(30r−1).S=\sum_{r=1}^{30} \frac{r^2\binom{30}{r}^2}{\binom{30}{r-1}}.S=∑r=130​(r−130​)r2(r30​)2​. We are given that S=α⋅229S=\alpha\cdot 2^{29}S=α⋅229 and we must find α\alphaα.

  2. First simplify the summand. Use (30r)=30!r!(30−r)!,(30r−1)=30!(r−1)!(31−r)!.\binom{30}{r}=\frac{30!}{r!(30-r)!},\qquad \binom{30}{r-1}=\frac{30!}{(r-1)!(31-r)!}.(r30​)=r!(30−r)!30!​,(r−130​)=(r−1)!(31−r)!30!​. Then (30r)(30r−1)=30−r+1r=31−rr.\frac{\binom{30}{r}}{\binom{30}{r-1}}=\frac{30-r+1}{r}=\frac{31-r}{r}.(r−130​)(r30​)​=r30−r+1​=r31−r​. So,

=r2(30r)⋅(30r)(30r−1)=r2(30r)⋅31−rr.= r^2\binom{30}{r}\cdot \frac{\binom{30}{r}}{\binom{30}{r-1}} = r^2\binom{30}{r}\cdot \frac{31-r}{r}.=r2(r30​)⋅(r−130​)(r30​)​=r2(r30​)⋅r31−r​.

Hence, r2(30r)2(30r−1)=r(31−r)(30r).\frac{r^2\binom{30}{r}^2}{\binom{30}{r-1}}=r(31-r)\binom{30}{r}.(r−130​)r2(r30​)2​=r(31−r)(r30​). Therefore S=∑r=130r(31−r)(30r).S=\sum_{r=1}^{30} r(31-r)\binom{30}{r}.S=∑r=130​r(31−r)(r30​).

  1. Expand: r(31−r)=31r−r2.r(31-r)=31r-r^2.r(31−r)=31r−r2. Thus S=31∑r=130r(30r)−∑r=130r2(30r).S=31\sum_{r=1}^{30} r\binom{30}{r}-\sum_{r=1}^{30} r^2\binom{30}{r}.S=31∑r=130​r(r30​)−∑r=130​r2(r30​). So we need the standard sums ∑r=0nr(nr)=n2n−1,\sum_{r=0}^{n} r\binom{n}{r}=n2^{n-1},∑r=0n​r(rn​)=n2n−1, ∑r=0nr(r−1)(nr)=n(n−1)2n−2.\sum_{r=0}^{n} r(r-1)\binom{n}{r}=n(n-1)2^{n-2}.∑r=0n​r(r−1)(rn​)=n(n−1)2n−2. Also, r2=r(r−1)+r,r^2=r(r-1)+r,r2=r(r−1)+r, so
  1. Put n=30n=30n=30. Then ∑r=030r(30r)=30⋅229,\sum_{r=0}^{30} r\binom{30}{r}=30\cdot 2^{29},∑r=030​r(r30​)=30⋅229, and ∑r=030r2(30r)=30⋅29⋅228+30⋅229.\sum_{r=0}^{30} r^2\binom{30}{r}=30\cdot 29\cdot 2^{28}+30\cdot 2^{29}.∑r=030​r2(r30​)=30⋅29⋅228+30⋅229. Since the r=0r=0r=0 term is 000, the same formulas apply for summation from 111 to 303030.

  2. Substitute into SSS: S=31(30⋅229)−(30⋅29⋅228+30⋅229).S=31(30\cdot 2^{29})-\left(30\cdot 29\cdot 2^{28}+30\cdot 2^{29}\right).S=31(30⋅229)−(30⋅29⋅228+30⋅229). Write everything in terms of 2282^{28}228: 31⋅30⋅229=31⋅30⋅2⋅228=1860⋅228,31\cdot 30\cdot 2^{29}=31\cdot 30\cdot 2\cdot 2^{28}=1860\cdot 2^{28},31⋅30⋅229=31⋅30⋅2⋅228=1860⋅228, 30⋅29⋅228=870⋅228,30\cdot 29\cdot 2^{28}=870\cdot 2^{28},30⋅29⋅228=870⋅228, 30⋅229=60⋅228.30\cdot 2^{29}=60\cdot 2^{28}.30⋅229=60⋅228. Therefore S=(1860−870−60)228=930⋅228.S=(1860-870-60)2^{28}=930\cdot 2^{28}.S=(1860−870−60)228=930⋅228. Now, 930⋅228=465⋅229.930\cdot 2^{28}=465\cdot 2^{29}.930⋅228=465⋅229. Hence α=465.\alpha=465.α=465.

  3. Comparison with stored answer: Stored correct answer = 465465465. Our derived answer also equals 465465465, so they agree.

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