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Sequences and Series question

2010 · Shift 1 · Q34
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Sequences and Series question

2010 · Shift 1 · Q34

JEE AdvancedMathematicsSequences and SeriesNumerical+4 / −1
Let Sk{S_k}Sk​= 1, 2,....., 100, denote the sum of the infinite geometric series whose first term is  k−1k !\,{{k - 1} \over {k\,!}}k!k−1​ and the common ratio is 1k{1 \over k}k1​. Then the value of 1002100!  +  ∑k=1100∣(k2−3k+1)  Sk∣  {{{{100}^2}} \over {100!}}\,\, + \,\,\sum\limits_{k = 1}^{100} {\left| {({k^2} - 3k + 1)\,\,{S_k}} \right|\,\,}100!1002​+k=1∑100​​(k2−3k+1)Sk​​ is
Numerical answer
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Correct answer: 3

Step 1: Calculate the sum of the infinite geometric series, SkS_kSk​.

The given infinite geometric series has the first term a=k−1k!a = \frac{k-1}{k!}a=k!k−1​ and the common ratio r=1kr = \frac{1}{k}r=k1​. The sum is denoted by SkS_kSk​.

Case 1: k = 1 For k=1k=1k=1, the first term is a=1−11!=0a = \frac{1-1}{1!} = 0a=1!1−1​=0. The common ratio is r=11=1r = \frac{1}{1} = 1r=11​=1. Since the first term is 0, the series is 0+0+0+…0 + 0 + 0 + \dots0+0+0+…, and its sum is S1=0S_1 = 0S1​=0.

Case 2: k > 1 (i.e., k = 2, 3, ..., 100) For these values of kkk, the common ratio ∣r∣=1k<1|r| = \frac{1}{k} < 1∣r∣=k1​<1. Thus, the sum of the infinite geometric series converges and is given by the formula Sk=a1−rS_k = \frac{a}{1-r}Sk​=1−ra​. Sk=k−1k!1−1k=k−1k!k−1k=k−1k!×kk−1=kk!S_k = \frac{\frac{k-1}{k!}}{1 - \frac{1}{k}} = \frac{\frac{k-1}{k!}}{\frac{k-1}{k}} = \frac{k-1}{k!} \times \frac{k}{k-1} = \frac{k}{k!}Sk​=1−k1​k!k−1​​=kk−1​k!k−1​​=k!k−1​×k−1k​=k!k​ Since k!=k×(k−1)!k! = k \times (k-1)!k!=k×(k−1)!, we have: Sk=kk(k−1)!=1(k−1)!S_k = \frac{k}{k(k-1)!} = \frac{1}{(k-1)!}Sk​=k(k−1)!k​=(k−1)!1​ So, we have S1=0S_1 = 0S1​=0 and Sk=1(k−1)!S_k = \frac{1}{(k-1)!}Sk​=(k−1)!1​ for k≥2k \ge 2k≥2.

Step 2: Evaluate the expression inside the summation.

We need to evaluate the sum ∑k=1100∣(k2−3k+1) Sk∣\sum\limits_{k = 1}^{100} {\left| {({k^2} - 3k + 1)\,{S_k}} \right|}k=1∑100​​(k2−3k+1)Sk​​. Let's analyze the term inside the absolute value, Tk=(k2−3k+1)SkT_k = (k^2 - 3k + 1)S_kTk​=(k2−3k+1)Sk​.

  • For k=1k=1k=1: T1=(12−3(1)+1)S1=(−1)(0)=0T_1 = (1^2 - 3(1) + 1)S_1 = (-1)(0) = 0T1​=(12−3(1)+1)S1​=(−1)(0)=0.
  • For k≥2k \ge 2k≥2: Tk=(k2−3k+1)1(k−1)!T_k = (k^2 - 3k + 1) \frac{1}{(k-1)!}Tk​=(k2−3k+1)(k−1)!1​.

To handle the absolute value, let's determine the sign of the quadratic factor k2−3k+1k^2 - 3k + 1k2−3k+1. The roots of the equation x2−3x+1=0x^2 - 3x + 1 = 0x2−3x+1=0 are x=3±9−42=3±52x = \frac{3 \pm \sqrt{9-4}}{2} = \frac{3 \pm \sqrt{5}}{2}x=23±9−4​​=23±5​​. Numerically, these roots are approximately 0.38 and 2.62. The parabola opens upwards, so the expression is negative between the roots.

  • For k=1k=1k=1, 12−3(1)+1=−1<01^2 - 3(1) + 1 = -1 < 012−3(1)+1=−1<0.
  • For k=2k=2k=2, 22−3(2)+1=4−6+1=−1<02^2 - 3(2) + 1 = 4 - 6 + 1 = -1 < 022−3(2)+1=4−6+1=−1<0.
  • For k=3k=3k=3, 32−3(3)+1=9−9+1=1>03^2 - 3(3) + 1 = 9 - 9 + 1 = 1 > 032−3(3)+1=9−9+1=1>0. For all k≥3k \ge 3k≥3, the expression k2−3k+1k^2 - 3k + 1k2−3k+1 is positive.

Step 3: Calculate the summation.

We can split the summation based on the sign of the term: ∑k=1100∣Tk∣=∣T1∣+∣T2∣+∑k=3100∣Tk∣\sum\limits_{k = 1}^{100} |T_k| = |T_1| + |T_2| + \sum\limits_{k = 3}^{100} |T_k|k=1∑100​∣Tk​∣=∣T1​∣+∣T2​∣+k=3∑100​∣Tk​∣

  • ∣T1∣=∣0∣=0|T_1| = |0| = 0∣T1​∣=∣0∣=0.
  • ∣T2∣=∣(22−3(2)+1)S2∣=∣(−1)1(2−1)!∣=∣−1×11!∣=1|T_2| = |(2^2 - 3(2) + 1)S_2| = |(-1) \frac{1}{(2-1)!}| = |-1 \times \frac{1}{1!}| = 1∣T2​∣=∣(22−3(2)+1)S2​∣=∣(−1)(2−1)!1​∣=∣−1×1!1​∣=1.
  • For k≥3k \ge 3k≥3, since both (k2−3k+1)(k^2 - 3k + 1)(k2−3k+1) and SkS_kSk​ are positive, ∣Tk∣=Tk|T_k| = T_k∣Tk​∣=Tk​.

The summation becomes: 0+1+∑k=3100(k2−3k+1)1(k−1)!0 + 1 + \sum\limits_{k = 3}^{100} (k^2 - 3k + 1) \frac{1}{(k-1)!}0+1+k=3∑100​(k2−3k+1)(k−1)!1​ Let's simplify the general term for k≥3k \ge 3k≥3 by rewriting the numerator: k2−3k+1=k2−2k+1−k=(k−1)2−kk^2 - 3k + 1 = k^2 - 2k + 1 - k = (k-1)^2 - kk2−3k+1=k2−2k+1−k=(k−1)2−k So, the term is: (k−1)2−k(k−1)!=(k−1)2(k−1)!−k(k−1)!=k−1(k−2)!−k(k−1)!\frac{(k-1)^2 - k}{(k-1)!} = \frac{(k-1)^2}{(k-1)!} - \frac{k}{(k-1)!} = \frac{k-1}{(k-2)!} - \frac{k}{(k-1)!}(k−1)!(k−1)2−k​=(k−1)!(k−1)2​−(k−1)!k​=(k−2)!k−1​−(k−1)!k​ This is a telescoping series. Let ak=k(k−1)!a_k = \frac{k}{(k-1)!}ak​=(k−1)!k​. Then the term is ak−1−aka_{k-1} - a_kak−1​−ak​.

The sum is: ∑k=3100(ak−1−ak)=(a2−a3)+(a3−a4)+⋯+(a99−a100)=a2−a100\sum\limits_{k = 3}^{100} (a_{k-1} - a_k) = (a_2 - a_3) + (a_3 - a_4) + \dots + (a_{99} - a_{100}) = a_2 - a_{100}k=3∑100​(ak−1​−ak​)=(a2​−a3​)+(a3​−a4​)+⋯+(a99​−a100​)=a2​−a100​ Now, we calculate a2a_2a2​ and a100a_{100}a100​: a2=2(2−1)!=21!=2a_2 = \frac{2}{(2-1)!} = \frac{2}{1!} = 2a2​=(2−1)!2​=1!2​=2 a100=100(100−1)!=10099!a_{100} = \frac{100}{(100-1)!} = \frac{100}{99!}a100​=(100−1)!100​=99!100​ So, ∑k=3100Tk=2−10099!\sum\limits_{k = 3}^{100} T_k = 2 - \frac{100}{99!}k=3∑100​Tk​=2−99!100​.

Putting it all together, the value of the full summation is: ∑k=1100∣Tk∣=1+(2−10099!)=3−10099!\sum\limits_{k = 1}^{100} |T_k| = 1 + \left(2 - \frac{100}{99!}\right) = 3 - \frac{100}{99!}k=1∑100​∣Tk​∣=1+(2−99!100​)=3−99!100​

Step 4: Calculate the final value of the expression.

The entire expression is: E=1002100!+∑k=1100∣(k2−3k+1)Sk∣E = \frac{100^2}{100!} + \sum\limits_{k = 1}^{100} |(k^2 - 3k + 1)S_k|E=100!1002​+k=1∑100​∣(k2−3k+1)Sk​∣. First, let's simplify the term 1002100!\frac{100^2}{100!}100!1002​: 1002100!=100×100100×99!=10099!\frac{100^2}{100!} = \frac{100 \times 100}{100 \times 99!} = \frac{100}{99!}100!1002​=100×99!100×100​=99!100​ Now, substitute the results from the previous steps: E=10099!+(3−10099!)=3E = \frac{100}{99!} + \left(3 - \frac{100}{99!}\right) = 3E=99!100​+(3−99!100​)=3

The final value of the expression is 3.

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