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STD 11 - 8. sequence and series — Maths STD 11 Science — Question

CBSE BoardEnglish MediumSTD 11 ScienceMathsSTD 11 - 8. sequence and series1 Mark
MCQ
Let $a , b , c$ be in arithmetic progression. Let the centroid of the triangle with vertices $( a , c ),(2, b)$ and $(a, b)$ be $\left(\frac{10}{3}, \frac{7}{3}\right)$. If $\alpha, \beta$ are the roots of the equation $ax ^{2}+ bx +1=0$, then the value of $\alpha^{2}+\beta^{2}-\alpha \beta$ is ....... .
  • A
    $\frac{71}{256}$
  • B
    $\frac{69}{256}$
  • C
    $-\frac{69}{256}$
  • ✓
    $-\frac{71}{256}$
✓

Answer

Correct option: D.
$-\frac{71}{256}$
d
$\frac{a+2+a}{3}=\frac{10}{3}$

$a=4$

and $\frac{c+b+b}{3}=\frac{7}{3}$

$c+2 b=7$

also $2 b=a+c$

$2 b-a+2 b=7$

$b=\frac{11}{4}$

now $4 x ^{2}+\frac{11}{4} x +1=0 (0=\alpha \,And \, \beta)$

$\alpha^{2}+\beta^{2}-\alpha \beta=(\alpha+\beta)^{2}-3 \alpha \beta$

$=\left(\frac{-11}{16}\right)^{2}-3\left(\frac{1}{4}\right)$

$=\frac{121}{256}-\frac{3}{4}=\frac{-71}{256}$

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