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Sequences and Series question

2024 · 30 Jan · Shift 1 · Q58
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  5. /2024 · 30 Jan · Shift 1 · Q58

Sequences and Series question

2024 · 30 Jan · Shift 1 · Q58

JEE MainMathematicsSequences and SeriesNumerical+4 / −1
Let α=12+42+82+132+192+262+…\alpha=1^2+4^2+8^2+13^2+19^2+26^2+\ldotsα=12+42+82+132+192+262+… upto 10 terms and β=∑n=110n4\beta=\sum_{n=1}^{10} n^4β=∑n=110​n4. If 4α−β=55k+404 \alpha-\beta=55 k+404α−β=55k+40, then k\mathrm{k}k is equal to ‾\underline{\hspace{2cm}}​.
Numerical answer
View written solutionFree

Correct answer: 353

  1. Identify the sequence in α\alphaα

The terms whose squares appear are: 1,4,8,13,19,26,…1,4,8,13,19,26,\ldots1,4,8,13,19,26,…

Their consecutive differences are: 3,4,5,6,7,…3,4,5,6,7,\ldots3,4,5,6,7,…

So the base sequence is quadratic in nnn. Let its nnnth term be ana_nan​.

Since a1=1a_1=1a1​=1 and differences increase by 111, we recognize an=1+∑r=1n−1(r+2)a_n=1+\sum_{r=1}^{n-1}(r+2)an​=1+∑r=1n−1​(r+2)

Now, an=1+∑r=1n−1r+2(n−1)a_n=1+\sum_{r=1}^{n-1}r+2(n-1)an​=1+∑r=1n−1​r+2(n−1) =1+(n−1)n2+2(n−1)=1+\frac{(n-1)n}{2}+2(n-1)=1+2(n−1)n​+2(n−1) =1+n2−n2+2n−2=1+\frac{n^2-n}{2}+2n-2=1+2n2−n​+2n−2 =n2+3n−22=\frac{n^2+3n-2}{2}=2n2+3n−2​

Check:

  • n=1n=1n=1: 1+3−22=1\frac{1+3-2}{2}=121+3−2​=1
  • n=2n=2n=2: 4+6−22=4\frac{4+6-2}{2}=424+6−2​=4
  • n=3n=3n=3: 9+9−22=8\frac{9+9-2}{2}=829+9−2​=8

Hence, an=n2+3n−22a_n=\frac{n^2+3n-2}{2}an​=2n2+3n−2​

Therefore, α=∑n=110an2=∑n=110(n2+3n−22)2\alpha=\sum_{n=1}^{10} a_n^2=\sum_{n=1}^{10}\left(\frac{n^2+3n-2}{2}\right)^2α=∑n=110​an2​=∑n=110​(2n2+3n−2​)2

  1. Compute 4α4\alpha4α

Since 4α=∑n=110(n2+3n−2)24\alpha=\sum_{n=1}^{10}(n^2+3n-2)^24α=∑n=110​(n2+3n−2)2

Expand: (n2+3n−2)2=n4+6n3+5n2−12n+4(n^2+3n-2)^2=n^4+6n^3+5n^2-12n+4(n2+3n−2)2=n4+6n3+5n2−12n+4

So, 4α=∑n=110(n4+6n3+5n2−12n+4)4\alpha=\sum_{n=1}^{10}\left(n^4+6n^3+5n^2-12n+4\right)4α=∑n=110​(n4+6n3+5n2−12n+4)

Given β=∑n=110n4\beta=\sum_{n=1}^{10}n^4β=∑n=110​n4

Thus, 4α−β=∑n=110(6n3+5n2−12n+4)4\alpha-\beta=\sum_{n=1}^{10}(6n^3+5n^2-12n+4)4α−β=∑n=110​(6n3+5n2−12n+4)

  1. Use standard summation formulas

For n=1n=1n=1 to 101010, ∑n=10⋅112=55\sum n=\frac{10\cdot 11}{2}=55∑n=210⋅11​=55 ∑n2=10⋅11⋅216=385\sum n^2=\frac{10\cdot 11\cdot 21}{6}=385∑n2=610⋅11⋅21​=385 ∑n3=(10⋅112)2=552=3025\sum n^3=\left(\frac{10\cdot 11}{2}\right)^2=55^2=3025∑n3=(210⋅11​)2=552=3025 ∑4=4⋅10=40\sum 4=4\cdot 10=40∑4=4⋅10=40

Therefore, 4α−β=6∑n3+5∑n2−12∑n+∑44\alpha-\beta=6\sum n^3+5\sum n^2-12\sum n+\sum 44α−β=6∑n3+5∑n2−12∑n+∑4 =6(3025)+5(385)−12(55)+40=6(3025)+5(385)-12(55)+40=6(3025)+5(385)−12(55)+40 =18150+1925−660+40=18150+1925-660+40=18150+1925−660+40 =19455=19455=19455

  1. Compare with the given form

We are told 4α−β=55k+404\alpha-\beta=55k+404α−β=55k+40

So, 55k+40=1945555k+40=1945555k+40=19455 55k=1941555k=1941555k=19415 k=1941555=353k=\frac{19415}{55}=353k=5519415​=353

  1. Final answer

353\boxed{353}353​

The derived answer matches the stored correct answer.

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