JEE MainMathematicsBinomial TheoremMCQ+4 / −1
- A11
- B20
- C24
- D15
View written solutionFree
Correct answer: B
- Rewrite the summand
Given
first simplify the factor:
So the sum becomes
Let Then as goes from to , goes from to . Also, . Hence
Now split the sum:
- Evaluate the first sum
Using we get
Therefore,
- Evaluate the second sum using the binomial theorem
Consider
Take :
We need
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**.More from Binomial Theorem
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