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Matrices and Determinants question

2025 · 29 Jan · Shift 2 · Q31
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Matrices and Determinants question

2025 · 29 Jan · Shift 2 · Q31

JEE MainMathematicsMatrices and DeterminantsMCQ+4 / −1
Let α,β (αeqβ)\alpha, \beta \ (\alpha eq \beta)α,β (αeqβ) be the values of mmm, for which the equations x+y+z=1x+y+z=1x+y+z=1, x+2y+4z=mx+2y+4z=mx+2y+4z=m and x+4y+10z=m2x+4y+10z=m^2x+4y+10z=m2 have infinitely many solutions. Then the value of ∑n=110(nα+nβ)\sum\limits_{n=1}^{10} (n^{\alpha}+n^{\beta})n=1∑10​(nα+nβ) is equal to :
  1. A
    3410
  2. B
    560
  3. C
    3080
  4. D
    440
View written solutionFree

Correct answer: D

  1. For the system to have infinitely many solutions, the three equations must be consistent and dependent.

The equations are: x+y+z=1x+y+z=1x+y+z=1 x+2y+4z=mx+2y+4z=mx+2y+4z=m x+4y+10z=m2x+4y+10z=m^2x+4y+10z=m2

So the augmented matrix must have rank less than 3, i.e. the third row should be a linear combination of the first two, and the constants must satisfy the same relation.

  1. First check dependence of the coefficient rows:

Let R1=(1,1,1),R2=(1,2,4),R3=(1,4,10)R_1=(1,1,1),\quad R_2=(1,2,4),\quad R_3=(1,4,10)R1​=(1,1,1),R2​=(1,2,4),R3​=(1,4,10) Assume R3=aR1+bR2R_3=aR_1+bR_2R3​=aR1​+bR2​ Then comparing coordinates: a+b=1a+b=1a+b=1 a+2b=4a+2b=4a+2b=4 a+4b=10a+4b=10a+4b=10

From the first two equations: (a+2b)−(a+b)=4−1⇒b=3(a+2b)-(a+b)=4-1 \Rightarrow b=3(a+2b)−(a+b)=4−1⇒b=3 Then a=1−b=1−3=−2a=1-b=1-3=-2a=1−b=1−3=−2 Check third: a+4b=−2+12=10a+4b=-2+12=10a+4b=−2+12=10 which is true.

Hence, R3=−2R1+3R2R_3=-2R_1+3R_2R3​=−2R1​+3R2​ So for consistency with infinitely many solutions, the constants must also satisfy: m2=−2(1)+3(m)m^2=-2(1)+3(m)m2=−2(1)+3(m) That is, m2=3m−2m^2=3m-2m2=3m−2 m2−3m+2=0m^2-3m+2=0m2−3m+2=0 (m−1)(m−2)=0(m-1)(m-2)=0(m−1)(m−2)=0

Thus, α=1,β=2\alpha=1,\quad \beta=2α=1,β=2 (up to order).

  1. Now compute ∑n=110(nα+nβ)=∑n=110(n+n2)\sum_{n=1}^{10}(n^{\alpha}+n^{\beta})=\sum_{n=1}^{10}(n+n^2)∑n=110​(nα+nβ)=∑n=110​(n+n2)

Using standard formulas: ∑n=110n=10⋅112=55\sum_{n=1}^{10} n=\frac{10\cdot 11}{2}=55∑n=110​n=210⋅11​=55 ∑n=110n2=10⋅11⋅216=385\sum_{n=1}^{10} n^2=\frac{10\cdot 11\cdot 21}{6}=385∑n=110​n2=610⋅11⋅21​=385

Therefore, ∑n=110(n+n2)=55+385=440\sum_{n=1}^{10}(n+n^2)=55+385=440∑n=110​(n+n2)=55+385=440

  1. Hence the correct option is: 440\boxed{440}440​ which is Option D.
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