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Binomial Theorem question

2025 · 2 Apr · Shift 1 · Q44
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  5. /2025 · 2 Apr · Shift 1 · Q44

Binomial Theorem question

2025 · 2 Apr · Shift 1 · Q44

JEE MainMathematicsBinomial TheoremMCQ+4 / −1
The term independent of xxx in the expansion of ((x+1)(x2/3+1−x1/3)−(x−1)(x−x1/2))10,x>1\left(\frac{(x+1)}{\left(x^{2 / 3}+1-x^{1 / 3}\right)}-\frac{(x-1)}{\left(x-x^{1 / 2}\right)}\right)^{10}, x\gt 1((x2/3+1−x1/3)(x+1)​−(x−x1/2)(x−1)​)10,x>1, is :
  1. A
    240
  2. B
    120
  3. C
    150
  4. D
    210
View written solutionFree

Correct answer: D

  1. Simplify the expression inside the bracket

We need the constant term in

(x+1x2/3+1−x1/3−x−1x−x1/2)10,x>1.\left(\frac{x+1}{x^{2/3}+1-x^{1/3}}-\frac{x-1}{x-x^{1/2}}\right)^{10}, \qquad x>1.(x2/3+1−x1/3x+1​−x−x1/2x−1​)10,x>1.

Let

t=x1/3.t=x^{1/3}.t=x1/3. Then x=t3x=t^3x=t3 and

x2/3+1−x1/3=t2+1−t=t2−t+1.x^{2/3}+1-x^{1/3}=t^2+1-t=t^2-t+1.x2/3+1−x1/3=t2+1−t=t2−t+1.

Also,

x+1=t3+1=(t+1)(t2−t+1).x+1=t^3+1=(t+1)(t^2-t+1).x+1=t3+1=(t+1)(t2−t+1).

Hence,

x+1x2/3+1−x1/3=(t+1)(t2−t+1)t2−t+1=t+1=x1/3+1.\frac{x+1}{x^{2/3}+1-x^{1/3}}=\frac{(t+1)(t^2-t+1)}{t^2-t+1}=t+1=x^{1/3}+1.x2/3+1−x1/3x+1​=t2−t+1(t+1)(t2−t+1)​=t+1=x1/3+1.

Now simplify the second fraction:

x−1x−x1/2=x−1x(1−x−1/2).\frac{x-1}{x-x^{1/2}}=\frac{x-1}{x(1-x^{-1/2})}.x−x1/2x−1​=x(1−x−1/2)x−1​.

A better factorization is:

x−1=(x−1)(x+1),x-1=(\sqrt{x}-1)(\sqrt{x}+1),x−1=(x​−1)(x​+1), x−x1/2=x(x−1).x-x^{1/2}=\sqrt{x}(\sqrt{x}-1).x−x1/2=x​(x​−1).

So,

x−1x−x1/2=(x−1)(x+1)x(x−1)=x+1x=1+x−1/2.\frac{x-1}{x-x^{1/2}}=\frac{(\sqrt{x}-1)(\sqrt{x}+1)}{\sqrt{x}(\sqrt{x}-1)}=\frac{\sqrt{x}+1}{\sqrt{x}}=1+x^{-1/2}.x−x1/2x−1​=x​(x​−1)(x​−1)(x​+1)​=x​x​+1​=1+x−1/2.

Therefore the bracket becomes

(x1/3+1−(1+x−1/2))10=(x1/3−x−1/2)10.\left(x^{1/3}+1-(1+x^{-1/2})\right)^{10}=(x^{1/3}-x^{-1/2})^{10}.(x1/3+1−(1+x−1/2))10=(x1/3−x−1/2)10.


  1. Rewrite for binomial expansion

(x1/3−x−1/2)10.(x^{1/3}-x^{-1/2})^{10}.(x1/3−x−1/2)10.

General term is

Tr+1=(10r)(x1/3)10−r(−x−1/2)rT_{r+1}=\binom{10}{r}(x^{1/3})^{10-r}(-x^{-1/2})^rTr+1​=(r10​)(x1/3)10−r(−x−1/2)r

=(10r)(−1)rx10−r3−r2.=\binom{10}{r}(-1)^r x^{\frac{10-r}{3}-\frac r2}.=(r10​)(−1)rx310−r​−2r​.

Power of xxx is

10−r3−r2=20−2r−3r6=20−5r6.\frac{10-r}{3}-\frac r2=\frac{20-2r-3r}{6}=\frac{20-5r}{6}.310−r​−2r​=620−2r−3r​=620−5r​.

For the term independent of xxx, set exponent =0=0=0:

20−5r6=0\frac{20-5r}{6}=0620−5r​=0 20−5r=020-5r=020−5r=0 r=4.r=4.r=4.


  1. Find the constant term

For r=4r=4r=4,

T5=(104)(−1)4=(104)=210.T_5=\binom{10}{4}(-1)^4=\binom{10}{4}=210.T5​=(410​)(−1)4=(410​)=210.

So the term independent of xxx is

210.\boxed{210}. 210​.


  1. Compare with stored answer

Stored correct answer: D

Our derived answer: D (210)

They agree.

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