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Sequences and Series question

2024 · 5 Apr · Shift 1 · Q42
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  5. /2024 · 5 Apr · Shift 1 · Q42

Sequences and Series question

2024 · 5 Apr · Shift 1 · Q42

JEE MainMathematicsSequences and SeriesMCQ+4 / −1
If 11+2+12+3+…+199+100=m\frac{1}{\sqrt{1}+\sqrt{2}}+\frac{1}{\sqrt{2}+\sqrt{3}}+\ldots+\frac{1}{\sqrt{99}+\sqrt{100}}=m1​+2​1​+2​+3​1​+…+99​+100​1​=m and 11⋅2+12⋅3+…+199⋅100=n\frac{1}{1 \cdot 2}+\frac{1}{2 \cdot 3}+\ldots+\frac{1}{99 \cdot 100}=\mathrm{n}1⋅21​+2⋅31​+…+99⋅1001​=n, then the point (m,n)(\mathrm{m}, \mathrm{n})(m,n) lies on the line
  1. A
    11(x−1)−100y=011(x-1)-100 y=011(x−1)−100y=0
  2. B
    11x−100y=011 x-100 y=011x−100y=0
  3. C
    11(x−1)−100(y−2)=011(x-1)-100(y-2)=011(x−1)−100(y−2)=0
  4. D
    11(x−2)−100(y−1)=011(x-2)-100(y-1)=011(x−2)−100(y−1)=0
View written solutionFree

Correct answer: B

  1. Find mmm

We have

m=∑k=1991k+k+1. m=\sum_{k=1}^{99} \frac{1}{\sqrt{k}+\sqrt{k+1}}.m=k=1∑99​k​+k+1​1​.

Rationalize each term:

1k+k+1=k+1−k(k+1+k)(k+1−k)=k+1−k.\frac{1}{\sqrt{k}+\sqrt{k+1}}=\frac{\sqrt{k+1}-\sqrt{k}}{(\sqrt{k+1}+\sqrt{k})(\sqrt{k+1}-\sqrt{k})}=\sqrt{k+1}-\sqrt{k}.k​+k+1​1​=(k+1​+k​)(k+1​−k​)k+1​−k​​=k+1​−k​.

So,

m=∑k=199(k+1−k).m=\sum_{k=1}^{99}(\sqrt{k+1}-\sqrt{k}).m=k=1∑99​(k+1​−k​).

This is a telescoping series:

m=(2−1)+(3−2)+⋯+(10−99)=10−1=9.m=(\sqrt2-1)+(\sqrt3-\sqrt2)+\cdots +(10-\sqrt{99})=10-1=9.m=(2​−1)+(3​−2​)+⋯+(10−99​)=10−1=9.

Hence,

m=9.m=9.m=9.
  1. Find nnn

We have

n=∑k=1991k(k+1).n=\sum_{k=1}^{99}\frac{1}{k(k+1)}.n=k=1∑99​k(k+1)1​.

Use partial fractions:

1k(k+1)=1k−1k+1.\frac{1}{k(k+1)}=\frac{1}{k}-\frac{1}{k+1}.k(k+1)1​=k1​−k+11​.

Thus,

n=∑k=199(1k−1k+1).n=\sum_{k=1}^{99}\left(\frac{1}{k}-\frac{1}{k+1}\right).n=k=1∑99​(k1​−k+11​).

Again this telescopes:

n=(1−12)+(12−13)+⋯+(199−1100)=1−1100=99100.n=\left(1-\frac12\right)+\left(\frac12-\frac13\right)+\cdots+\left(\frac{1}{99}-\frac{1}{100}\right)=1-\frac{1}{100}=\frac{99}{100}.n=(1−21​)+(21​−31​)+⋯+(991​−1001​)=1−1001​=10099​.

Hence,

n=99100.n=\frac{99}{100}.n=10099​.
  1. Point (m,n)(m,n)(m,n)

So the point is

(m,n)=(9,99100).(m,n)=\left(9,\frac{99}{100}\right).(m,n)=(9,10099​).
  1. Check the options
  • A: 11(x−1)−100y=011(x-1)-100y=011(x−1)−100y=0

Substitute x=9,y=99100x=9, y=\frac{99}{100}x=9,y=10099​:

11(9−1)−100⋅99100=88−99=−11≠0.11(9-1)-100\cdot\frac{99}{100}=88-99=-11\neq 0.11(9−1)−100⋅10099​=88−99=−11=0.

So A is false.

  • B: 11x−100y=011x-100y=011x−100y=0
11⋅9−100⋅99100=99−99=0.11\cdot 9-100\cdot\frac{99}{100}=99-99=0.11⋅9−100⋅10099​=99−99=0.

So B is true.

  • C: 11(x−1)−100(y−2)=011(x-1)-100(y-2)=011(x−1)−100(y−2)=0
11(8)−100(99100−2)=88−(−101)=189≠0.11(8)-100\left(\frac{99}{100}-2\right)=88-(-101)=189\neq 0.11(8)−100(10099​−2)=88−(−101)=189=0.

So C is false.

  • D: 11(x−2)−100(y−1)=011(x-2)-100(y-1)=011(x−2)−100(y−1)=0
11(7)−100(99100−1)=77−(−1)=78≠0.11(7)-100\left(\frac{99}{100}-1\right)=77-(-1)=78\neq 0.11(7)−100(10099​−1)=77−(−1)=78=0.

So D is false.

Therefore, the correct option is

B.\boxed{\text{B}}.B​.
  1. Comparison with stored answer

Stored correct answer: B.

My derived answer also gives B, so they agree.

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