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Binomial Theorem question

2025 · 2 Apr · Shift 2 · Q42
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  5. /2025 · 2 Apr · Shift 2 · Q42

Binomial Theorem question

2025 · 2 Apr · Shift 2 · Q42

JEE MainMathematicsBinomial TheoremMCQ+4 / −1
If ∑r=010(10r+1−110r).11Cr+1=α11−11111010, then  α  is  equal  to:If\,\sum\limits_{r = 0}^{10} {({{{{10}^{r + 1}} - 1} \over {{{10}^r}}}).{}^{11}{C_{r + 1}} = {{{}_\alpha 11 - {{11}^{11}}} \over {{{10}^{10}}}},\,then\,\,\alpha \,\,is\,\,equal\,\,to:}Ifr=0∑10​(10r10r+1−1​).11Cr+1​=1010α​11−1111​,thenαisequalto:
  1. A
    11
  2. B
    20
  3. C
    24
  4. D
    15
View written solutionFree

Correct answer: B

  1. Rewrite the summand

Given ∑r=010(10r+1−110r)(11r+1)\sum_{r=0}^{10} \left(\frac{10^{r+1}-1}{10^r}\right)\binom{11}{r+1}∑r=010​(10r10r+1−1​)(r+111​)

first simplify the factor: 10r+1−110r=10−10−r.\frac{10^{r+1}-1}{10^r}=10-10^{-r}.10r10r+1−1​=10−10−r.

So the sum becomes S=∑r=010(10−110r)(11r+1).S=\sum_{r=0}^{10}\left(10-\frac1{10^r}\right)\binom{11}{r+1}.S=∑r=010​(10−10r1​)(r+111​).

Let k=r+1.k=r+1.k=r+1. Then as rrr goes from 000 to 101010, kkk goes from 111 to 111111. Also, r=k−1r=k-1r=k−1. Hence S=∑k=111(10−110k−1)(11k).S=\sum_{k=1}^{11}\left(10-\frac1{10^{k-1}}\right)\binom{11}{k}.S=∑k=111​(10−10k−11​)(k11​).

Now split the sum: S=10∑k=111(11k)−∑k=111110k−1(11k).S=10\sum_{k=1}^{11}\binom{11}{k}-\sum_{k=1}^{11}\frac{1}{10^{k-1}}\binom{11}{k}.S=10∑k=111​(k11​)−∑k=111​10k−11​(k11​).


  1. Evaluate the first sum

Using ∑k=011(11k)=211=2048,\sum_{k=0}^{11}\binom{11}{k}=2^{11}=2048,∑k=011​(k11​)=211=2048, we get ∑k=111(11k)=211−1=2047.\sum_{k=1}^{11}\binom{11}{k}=2^{11}-1=2047.∑k=111​(k11​)=211−1=2047.

Therefore, 10∑k=111(11k)=10(2047)=20470.10\sum_{k=1}^{11}\binom{11}{k}=10(2047)=20470.10∑k=111​(k11​)=10(2047)=20470.


  1. Evaluate the second sum using the binomial theorem

Consider ∑k=011(11k)xk=(1+x)11.\sum_{k=0}^{11}\binom{11}{k}x^k=(1+x)^{11}.∑k=011​(k11​)xk=(1+x)11.

Take x=110x=\frac{1}{10}x=101​: ∑k=011(11k)(110)k=(1+110)11=(1110)11.\sum_{k=0}^{11}\binom{11}{k}\left(\frac1{10}\right)^k=\left(1+\frac1{10}\right)^{11}=\left(\frac{11}{10}\right)^{11}.∑k=011​(k11​)(101​)k=(1+101​)11=(1011​)11.

We need ∑k=111110k−1(11k)=10∑k=111(11k)(110)k.\sum_{k=1}^{11}\frac{1}{10^{k-1}}\binom{11}{k}=10\sum_{k=1}^{11}\binom{11}{k}\left(\frac1{10}\right)^k.∑k=111​10k−11​(k11​)=10∑k=111​(k11​)(101​)k.

So,

=10\left[\left(\frac{11}{10}\right)^{11}-1\right].$$ --- 4. **Substitute back into $S$** Thus, $$S=20470-10\left[\left(\frac{11}{10}\right)^{11}-1\right].$$ Simplify: $$S=20470-10\left(\frac{11^{11}}{10^{11}}-1\right) =20470-\frac{11^{11}}{10^{10}}+10.$$ Hence $$S=20480-\frac{11^{11}}{10^{10}}.$$ Write $20480$ with denominator $10^{10}$: $$20480=\frac{20480\cdot 10^{10}}{10^{10}}.$$ Therefore, $$S=\frac{20480\cdot 10^{10}-11^{11}}{10^{10}}.$$ Comparing with $$S=\frac{\alpha\cdot 11-11^{11}}{10^{10}},$$ we get $$11\alpha=20480\cdot 10^{10}.$$ This comparison clearly suggests the printed expression is intended as $$\frac{\alpha-11^{11}}{10^{10}},$$ which is the standard form matching our simplification. Then $$\alpha=20480\cdot 10^{10}=2048\cdot 10^{11}=2^{11}\cdot 10^{11}=20^{11}.$$ So, $$\alpha=20^{11}.$$ Since the options are ordinary numbers, the intended question is asking for the base, i.e. $\alpha=20$. --- 5. **Check the options** Thus the correct option is: $$\boxed{20}$$ which is **Option B**.
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