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Mathematical Induction and Binomial Theorem question

2013 · Shift 1 · Q25
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  5. /2013 · Shift 1 · Q25

Mathematical Induction and Binomial Theorem question

2013 · Shift 1 · Q25

JEE AdvancedMathematicsMathematical Induction and Binomial TheoremNumerical+4 / −1
The coefficient of three consecutive terms of (1+x)n+5{\left( {1 + x} \right)^{n + 5}}(1+x)n+5 are in the ratio 5:10:14.5:10:14.5:10:14. Then nnn =
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Correct answer: 6

  1. Let the three consecutive binomial coefficients in (1+x)n+5(1+x)^{n+5}(1+x)n+5 be (n+5r−1), (n+5r), (n+5r+1)\binom{n+5}{r-1},\ \binom{n+5}{r},\ \binom{n+5}{r+1}(r−1n+5​), (rn+5​), (r+1n+5​) for some integer rrr.

    They are given to be in the ratio 5:10:14.5:10:14.5:10:14.

  2. So, (n+5r−1):(n+5r):(n+5r+1)=5:10:14.\binom{n+5}{r-1}:\binom{n+5}{r}:\binom{n+5}{r+1}=5:10:14.(r−1n+5​):(rn+5​):(r+1n+5​)=5:10:14.

  3. Use ratios of consecutive binomial coefficients: (Nr)(Nr−1)=N−r+1r,(Nr+1)(Nr)=N−rr+1\frac{\binom{N}{r}}{\binom{N}{r-1}}=\frac{N-r+1}{r}, \qquad \frac{\binom{N}{r+1}}{\binom{N}{r}}=\frac{N-r}{r+1}(r−1N​)(rN​)​=rN−r+1​,(rN​)(r+1N​)​=r+1N−r​ where N=n+5N=n+5N=n+5.

  4. From the given ratio, (Nr)(Nr−1)=105=2\frac{\binom{N}{r}}{\binom{N}{r-1}}=\frac{10}{5}=2(r−1N​)(rN​)​=510​=2 so N−r+1r=2\frac{N-r+1}{r}=2rN−r+1​=2 N−r+1=2rN-r+1=2rN−r+1=2r N+1=3rN+1=3rN+1=3r r=N+13.r=\frac{N+1}{3}.r=3N+1​.

  5. Also, (Nr+1)(Nr)=1410=75\frac{\binom{N}{r+1}}{\binom{N}{r}}=\frac{14}{10}=\frac{7}{5}(rN​)(r+1N​)​=1014​=57​ so N−rr+1=75\frac{N-r}{r+1}=\frac{7}{5}r+1N−r​=57​ 5(N−r)=7(r+1)5(N-r)=7(r+1)5(N−r)=7(r+1) 5N−5r=7r+75N-5r=7r+75N−5r=7r+7 5N−7=12r5N-7=12r5N−7=12r r=5N−712.r=\frac{5N-7}{12}.r=125N−7​.

  6. Equate the two expressions for rrr: N+13=5N−712\frac{N+1}{3}=\frac{5N-7}{12}3N+1​=125N−7​ 4(N+1)=5N−74(N+1)=5N-74(N+1)=5N−7 4N+4=5N−74N+4=5N-74N+4=5N−7 N=11.N=11.N=11.

  7. Since N=n+5N=n+5N=n+5, n+5=11n+5=11n+5=11 n=6.n=6.n=6.

  8. Therefore, the required integer is 6.\boxed{6}.6​.

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