Sign in
12thPass logo
New chatPYQ LibraryDoubtsRank report
Sign in to see Recents

Your guest activity stays on this device

Sign in to save progress →
Sign in

Quadratic Equation and Inequalities question

2019 · 8 Apr · Shift 2 · Q30
Guest · filters and generic practice availableBrowsing as a guest · PYQ filters and generic practice are available. Sign in only for personalised features and saved progress.
  1. PYQ Library
  2. /JEE Main
  3. /Mathematics
  4. /Quadratic Equation and Inequalities
  5. /2019 · 8 Apr · Shift 2 · Q30

Quadratic Equation and Inequalities question

2019 · 8 Apr · Shift 2 · Q30

JEE MainMathematicsQuadratic Equation and InequalitiesMCQ+4 / −1
The number of integral values of m for which the equation (1 + m2 )x2 – 2(1 + 3m)x + (1 + 8m) = 0 has no real root is :
  1. A
    2
  2. B
    infinitely many
  3. C
    1
  4. D
    3
View written solutionFree

Correct answer: B

  1. Consider the quadratic equation

(1+m2)x2−2(1+3m)x+(1+8m)=0.(1+m^2)x^2-2(1+3m)x+(1+8m)=0.(1+m2)x2−2(1+3m)x+(1+8m)=0.

For a quadratic equation ax2+bx+c=0ax^2+bx+c=0ax2+bx+c=0 to have no real root, we need:

  • a≠0a \neq 0a=0, and
  • discriminant D<0D<0D<0.

Here,

a=1+m2,b=−2(1+3m),c=1+8m.a=1+m^2,\quad b=-2(1+3m),\quad c=1+8m.a=1+m2,b=−2(1+3m),c=1+8m.

Since

1+m2>01+m^2>01+m2>0

for every integral mmm, the equation is always quadratic.

  1. Compute the discriminant:

D=b2−4ac.D=b^2-4ac.D=b2−4ac.

So,

D=[−2(1+3m)]2−4(1+m2)(1+8m).D=[-2(1+3m)]^2-4(1+m^2)(1+8m).D=[−2(1+3m)]2−4(1+m2)(1+8m).

That is,

D=4(1+3m)2−4(1+m2)(1+8m).D=4(1+3m)^2-4(1+m^2)(1+8m).D=4(1+3m)2−4(1+m2)(1+8m).

Factor out 444:

D=4[(1+3m)2−(1+m2)(1+8m)].D=4\left[(1+3m)^2-(1+m^2)(1+8m)\right].D=4[(1+3m)2−(1+m2)(1+8m)].

  1. Expand the terms inside the bracket:

(1+3m)2=1+6m+9m2.(1+3m)^2=1+6m+9m^2.(1+3m)2=1+6m+9m2.

Also,

(1+m2)(1+8m)=1+8m+m2+8m3.(1+m^2)(1+8m)=1+8m+m^2+8m^3.(1+m2)(1+8m)=1+8m+m2+8m3.

Therefore,

D=4[(1+6m+9m2)−(1+8m+m2+8m3)].D=4\left[(1+6m+9m^2)-(1+8m+m^2+8m^3)\right].D=4[(1+6m+9m2)−(1+8m+m2+8m3)].

Simplify:

D=4(−2m+8m2−8m3).D=4(-2m+8m^2-8m^3).D=4(−2m+8m2−8m3).

Factor further:

D=4[−2m(1−4m+4m2)]D=4[-2m(1-4m+4m^2)]D=4[−2m(1−4m+4m2)]

and since

1−4m+4m2=(2m−1)2,1-4m+4m^2=(2m-1)^2,1−4m+4m2=(2m−1)2,

we get

D=−8m(2m−1)2.D=-8m(2m-1)^2.D=−8m(2m−1)2.

  1. For no real roots, require

D<0.D<0.D<0.

Thus,

−8m(2m−1)2<0.-8m(2m-1)^2<0.−8m(2m−1)2<0.

Now, (2m−1)2≥0(2m-1)^2\ge 0(2m−1)2≥0 for all integers mmm, and it is zero only when m=12m=\tfrac12m=21​, which is not an integer. Hence for integral mmm,

(2m−1)2>0.(2m-1)^2>0.(2m−1)2>0.

So the sign of DDD depends only on −m-m−m.

Therefore,

D<0  ⟺  m>0.D<0 \iff m>0.D<0⟺m>0.

  1. Count integral values of mmm:

All positive integers satisfy the condition:

m=1,2,3,…m=1,2,3,\dotsm=1,2,3,…

Hence there are infinitely many integral values of mmm.

  1. Check options:
  • A: 222 ❌
  • B: infinitely many ✅
  • C: 111 ❌
  • D: 333 ❌

Therefore, the correct option is B.

PreviousNext

More from Quadratic Equation and Inequalities

  • Let p, q ∈ R. If 2 - 3​ is a root of the quadratic equation, x2 + px + q = 0, then :2019 · MCQ
  • If m is chosen in the quadratic equation (m2 + 1) x2 – 3x + (m2 + 1)2 = 0 such that the sum of its roots is greatest, then the absolute difference of the cubes of its roots is :-2019 · MCQ
  • If both the roots of the quadratic equation x2 − mx + 4 = 0 are real and distinct and they lie in the interval [1, 5], then m lies in the interval :2019 · MCQ
  • The number of all possible positive integral values of α for which the roots of the quadratic equation, 6x2 − 11x +α = 0 are rational numbers is :2019 · MCQ
  • If α and β are the roots of the quadratic equation, x2 + x sin θ- 2 sin θ= 0, θ∈(0,2π​), then (α−12+β−12).(α−β)24α12+β12​…2019 · MCQ
  • All the pairs (x, y) that satisfy the inequality 2sin2x−2sinx+5​.4sin2y1​≤1 also satisfy the equation2019 · MCQ
  • The number of real roots of the equation 5 + |2x – 1| = 2x (2x – 2) is2019 · MCQ
  • Consider the quadratic equation (c – 5)x2 – 2cx + (c – 4) = 0, c e 5. Let S be the set of all integral values of c for which one root of the equation lies in the interval (0, 2) and its other root lies in the interval (2, 3). Then the…2019 · MCQ