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Binomial Theorem question

2021 · 25 Jul · Shift 1 · Q44
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  5. /2021 · 25 Jul · Shift 1 · Q44

Binomial Theorem question

2021 · 25 Jul · Shift 1 · Q44

JEE MainMathematicsBinomial TheoremNumerical+4 / −1
The term independent of 'x' in the expansion of (x+1x2/3−x1/3+1−x−1x−x1/2)10{\left( {{{x + 1} \over {{x^{2/3}} - {x^{1/3}} + 1}} - {{x - 1} \over {x - {x^{1/2}}}}} \right)^{10}}(x2/3−x1/3+1x+1​−x−x1/2x−1​)10, where x eee 0, 1 is equal to ‾\underline{\hspace{2cm}}​.
Numerical answer
View written solutionFree

Correct answer: 210

  1. Simplify the expression inside the bracket

We need the constant term in

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

Let t=x1/3.t=x^{1/3}.t=x1/3. Then x=t3x=t^3x=t3 and x1/2=t3/2x^{1/2}=t^{3/2}x1/2=t3/2 is not convenient, so instead simplify each fraction directly using algebraic identities.


  1. First fraction

Use a2−a+1=a3+1a+1a^2-a+1=\frac{a^3+1}{a+1}a2−a+1=a+1a3+1​ with a=x1/3a=x^{1/3}a=x1/3. Since x2/3−x1/3+1=t2−t+1=t3+1t+1=x+1x1/3+1,x^{2/3}-x^{1/3}+1=t^2-t+1=\frac{t^3+1}{t+1}=\frac{x+1}{x^{1/3}+1},x2/3−x1/3+1=t2−t+1=t+1t3+1​=x1/3+1x+1​, we get

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


  1. Second fraction

Factor the denominator: x−x1/2=x1/2(x1/2−1).x-x^{1/2}=x^{1/2}(x^{1/2}-1).x−x1/2=x1/2(x1/2−1). Also, x−1=(x1/2−1)(x1/2+1).x-1=(x^{1/2}-1)(x^{1/2}+1).x−1=(x1/2−1)(x1/2+1). Hence

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


  1. Expression inside the bracket

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

So the given expression becomes

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


  1. General term

In the binomial expansion, Tr+1=(10r)(x1/3)10−r(−x−1/2)r.T_{r+1}=\binom{10}{r}(x^{1/3})^{10-r}(-x^{-1/2})^r.Tr+1​=(r10​)(x1/3)10−r(−x−1/2)r.

Thus the power of xxx in the general term is

10−r3−r2.\frac{10-r}{3}-\frac{r}{2}.310−r​−2r​.

For the term independent of xxx, set this exponent to 000:

10−r3−r2=0.\frac{10-r}{3}-\frac{r}{2}=0.310−r​−2r​=0.

Multiply by 666: 2(10−r)−3r=02(10-r)-3r=02(10−r)−3r=0 20−2r−3r=020-2r-3r=020−2r−3r=0 20−5r=020-5r=020−5r=0 r=4.r=4.r=4.


  1. Constant term

For r=4r=4r=4, T5=(104)(x1/3)6(−x−1/2)4.T_5=\binom{10}{4}(x^{1/3})^6(-x^{-1/2})^4.T5​=(410​)(x1/3)6(−x−1/2)4. Since the power of xxx is zero, the coefficient is

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


  1. Final answer

The term independent of xxx is

210.\boxed{210}.210​.

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