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Binomial Theorem question

2024 · 30 Jan · Shift 1 · Q55
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  5. /2024 · 30 Jan · Shift 1 · Q55

Binomial Theorem question

2024 · 30 Jan · Shift 1 · Q55

JEE MainMathematicsBinomial TheoremNumerical+4 / −1
 Number of integral terms in the expansion of {7(12)+11(16)}824 is equal to ‾. \text { Number of integral terms in the expansion of }\left\{7^{\left(\frac{1}{2}\right)}+11^{\left(\frac{1}{6}\right)}\right\}^{824} \text { is equal to \underline{\hspace{2cm}}. } Number of integral terms in the expansion of {7(21​)+11(61​)}824 is equal to ​. 
Numerical answer
View written solutionFree

Correct answer: 138

  1. General term of the binomial expansion

For (71/2+111/6)824,\left(7^{1/2}+11^{1/6}\right)^{824},(71/2+111/6)824, the general term is Tr+1=(824r)(71/2)824−r(111/6)r,T_{r+1}=\binom{824}{r}\left(7^{1/2}\right)^{824-r}\left(11^{1/6}\right)^r,Tr+1​=(r824​)(71/2)824−r(111/6)r, where r=0,1,2,…,824r=0,1,2,\dots,824r=0,1,2,…,824.

So, Tr+1=(824r) 7824−r2 11r6.T_{r+1}=\binom{824}{r}\,7^{\frac{824-r}{2}}\,11^{\frac{r}{6}}.Tr+1​=(r824​)72824−r​116r​.

  1. Condition for an integral term

A term will be integral if the exponents of both 777 and 111111 are integers.

So we need: 824−r2∈Z\frac{824-r}{2}\in \mathbb{Z}2824−r​∈Z and r6∈Z.\frac{r}{6}\in \mathbb{Z}.6r​∈Z.

  1. Solve the divisibility conditions

From r6∈Z,\frac{r}{6}\in \mathbb{Z},6r​∈Z, we get r≡0(mod6).r\equiv 0 \pmod{6}.r≡0(mod6).

If rrr is divisible by 666, then it is automatically even, hence 824−r824-r824−r is also even because 824824824 is even. Therefore, 824−r2\frac{824-r}{2}2824−r​ is automatically an integer.

So the only effective condition is: r=6kr=6kr=6k for some integer kkk with 0≤r≤824.0\le r\le 824.0≤r≤824.

Thus, 0≤6k≤824  ⟹  0≤k≤⌊8246⌋=137.0\le 6k\le 824 \implies 0\le k\le \left\lfloor \frac{824}{6}\right\rfloor =137.0≤6k≤824⟹0≤k≤⌊6824​⌋=137.

Hence, k=0,1,2,…,137,k=0,1,2,\dots,137,k=0,1,2,…,137, which gives 137−0+1=138137-0+1=138137−0+1=138 values.

  1. Number of integral terms

Therefore, the number of integral terms is 138.\boxed{138}.138​.

  1. Comparison with stored answer

Stored correct answer = 138138138.

My derived answer also is 138138138, so they agree.

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