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Nature Of Roots Depending Upon Coefficients And Discriminant - Practice Questions & MCQ

Edited By admin | Updated on Sep 18, 2023 18:34 AM | #JEE Main

Quick Facts

  • Nature of Roots, Relation Between Roots and Coefficient of Quadratic Equation is considered one of the most asked concept.

  • 86 Questions around this concept.

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If (1 + k) \tan ^2 x - 4 \tan x - 1 - k = 0 has real roots \tan x_1 and \tan x_2 , where \tan x_ 1 \neq \tan x_2  then

 The number of real solutions of the equation 3\left(x^2+\frac{1}{x^2}\right)-2\left(x+\frac{1}{x}\right)+5=0, is

Let the function f(x)=2 x^3+(2 p-7) x^2+3(2 p-9) x-6 have a maxima for some value of x<0 and a minima for some value of x>0. Then, the set of all values of p is

Let \lambda \neq 0 be a real number. Let \alpha, \beta be the roots of the equation 14 \mathrm{x}^2-31 \mathrm{x}+3 \lambda=0 and \alpha, \gamma be the roots of the equation 35 \mathrm{x}^2-53 \mathrm{x}+4 \lambda=0. Then \frac{3 \alpha}{\beta}\: and\: \frac{4 \alpha}{\gamma} are the roots of the equation
 

The number of real roots of the equation  \sqrt{x^2-4 x+3}+\sqrt{x^2-9}=\sqrt{4 x^2-14 x+6} , is

 

If for \mathrm{z}=\alpha+\mathrm{i} \beta,|\mathrm{z}+2|=\mathrm{z}+4(1+\mathrm{i}), then \alpha+\beta and \alpha\beta are the roots of the equation:

Let $S$ be the set of positive integral values of a for which $\frac{a^2+2(a+1) x+9 a+4}{x^2-8 x+32}<0, \forall x \in \mathbb{R}$. Then, the number of elements in $\mathrm{S}$ is:

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The sum of all the roots of the equation \left|x^{2}-8 x+15\right|-2 x+7=0 is:

If $\alpha, \beta$ are the roots of the equation, $x^2-x-1=0$ and $S_n=2023 \alpha^n+2024 \beta^n$, then:

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Let $\alpha$ and $\beta$ be the roots of the equation $\mathrm{px}^2+\mathrm{qx}-\mathrm{r}=0$, where $\mathrm{p} \neq 0$. If $\mathrm{p}, \mathrm{q}$ and $\mathrm{r}$ be the consecutive terms of a non constant G.P. and $\frac{1}{\alpha}+\frac{1}{\beta}=\frac{3}{4}$, then the value of $(\alpha-\beta)^2$ is :

Concepts Covered - 2

Nature of Roots

Let the quadratic equation is ax2 + bx + c = 0, (a,b,c  R)

D (called the discriminant of the equation) = b2 - 4ac

Roots of this equation are given by

\\\mathrm{x_1 = \frac{-b + \sqrt{D}}{2a} } \;\mathrm{and \;\;x_2 = \frac{-b - \sqrt{D}}{2a} }

 

i) if D < 0, then both roots are non-real (imaginary numbers), and the roots will be conjugate of each other, means if p + iq is one of          

   the roots then other root will be p - iq

 

ii) If D > 0, then roots will be real and distinct

 

iii) if roots D = 0, then roots will be real and equal, and they equal
\\\mathrm{x_1=x_2 = \frac{-b}{2a} } \\

 

Special cases of case ii (D > 0)

i) if a,b,c are rational numbers (Q) and

    If D is perfect square, then roots are rational

    If D is not perfect square than roots are irrational (in this case if p + \sqrt{q}   is one root of quadratic equation then other root will be p - \sqrt{q})

ii) If a = 1 and b and c are integers and  

    If D is perfect square, then roots are integers

    If D is not perfect square than roots are non-integrer values

Relation Between Roots and Coefficient of Quadratic Equation

Let ? and ? be two roots of a quadratic equation. So, we have

\\\mathrm{\alpha = \frac{-b-\sqrt{D}}{2a}} \\\mathrm{\beta = \frac{-b+\sqrt{D}}{2a}}

 

Sum of roots:

\\\mathrm{\alpha + \beta = \frac{-b - \sqrt{D}}{2a}+\frac{-b + \sqrt{D}}{2a}=\frac{-b}{a}}

Product of roots:

 \\\mathrm{\alpha \cdot \beta = \left ( \frac{-b - \sqrt{D}}{2a} \right )\cdot \left ( \frac{-b + \sqrt{D}}{2a} \right )} \\\mathrm{=\frac{b^2-D}{4a^2} = \frac{b^2-b^2+4ac}{4a^2} =\frac{4ac}{4a^2}=\frac{c}{a}}

 

The difference of root can also be found in the same way by manipulating the terms

\alpha - \beta = |\frac{\sqrt{D}}{a}|

 

 

Important Results

\\\mathrm{(i)\;\;\;\;\alpha^2+\beta^2=(\alpha+\beta)^2-2\alpha\beta}\\\mathrm{(ii)\;\;\;\alpha^2-\beta^2=(\alpha+\beta)(\alpha-\beta)}\\\mathrm{(iii)\;\;\alpha^3+\beta^3=(\alpha+\beta)^3-3\alpha\beta(\alpha+\beta)}\\\mathrm{(iv)\;\;\alpha^3-\beta^3=(\alpha-\beta)^3+3\alpha\beta(\alpha-\beta)}

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Nature of Roots
Relation Between Roots and Coefficient of Quadratic Equation

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