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Published on: 01/06/2021
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Questions + Answers key
Take MCQ Maths Test1.
If 2+i and 3-\(\sqrt{2}\) are roots of the equation x6-13x5+ 62x4-126x3+ 65x2+127x-140 = 0, find all roots.
2.
Solve the equation x3− 9x2+14x + 24 = 0 if it is given that two of its roots are in the ratio 3:2.
3.
Form the equation whose roots are the squares of the roots of the cubic equation x3+ ax2+ bx + c = 0.
4.
Solve the equation 3x3 - 16x2 + 23x - 6 = 0 if the product of two roots is 1.
5.
Find a polynomial equation of minimum degree with rational coefficients, having \(\sqrt{5}\)−\(\sqrt{3}\) as a root.
1.
Since the coefficients of the equation are all rational numbers, 2+i and 3-\(\sqrt{2}\) are roots, we get 2-i and 3+\(\sqrt{2}\) are also roots of the given equation. Thus (x-(2+i)), (x-(2-i)), (x-(3-\(\sqrt{2}\))) and (x-(3+\(\sqrt{2}\))) are factors. Thus their product.
((x-(2+i))(x-(2-i))(x-(3-\(\sqrt{2}\)))(x-(3+\(\sqrt{2}\))) is a factor of the given polynomial equation.
That is, (x2-4x+5)(x2-6x+7) is a factor. Dividing the given polynomial equation by this factor, we get the other factor as (x2-3x-4) which implies that 4 and −1 are the other two roots. Thus
2+i, 2-i, 3+\(\sqrt{2}\), 3-\(\sqrt{2}\), -1, and 4 are the roots of the given polynomial equation.
2.
Let ∝, β, ૪ be the roots of the equation
Given \(\frac { \alpha }{ \beta } =\frac { 3 }{ 2 } \Rightarrow 2\alpha =3\beta \Rightarrow \alpha =\frac { 3 }{ 2 } \beta \)
\(\therefore \frac { 3 }{ 2 } \beta ,\beta ,\gamma \) are the roots of the given equation
Then by Vieta's formula,
\(\frac { 3 }{ 2 } \beta +\beta +\gamma =\frac { -b }{ a } =\frac { -(-9) }{ 1 } =9\)
\(\frac { 5 }{ 2 } \beta +\gamma =9\Rightarrow \gamma =9-\frac { 5 }{ 2 } \beta \)
\(\Rightarrow \gamma =\frac { 18-5\beta }{ 2 } ...(2)\)
Also \(\frac { 3 }{ 2 } \beta (\beta )+\beta \gamma +\left( \frac { 3 }{ 2 } \beta \right) \gamma =\frac { c }{ a } =\frac { 14 }{ 1 } =14\)
\(\Rightarrow \frac { 3 }{ 2 } { \beta }^{ 2 }+\frac { 5 }{ 2 } \beta \left( \frac { 18-5\beta }{ 2 } \right) =14\ [using\ (2)]\)
\(\Rightarrow \frac { 3 }{ 2 } { \beta }^{ 2 }+\frac { 90\beta }{ 4 } -\frac { 25{ \beta }^{ 2 } }{ 4 } =14\)
Multiplying by \(4,6{ \beta }^{ 2 }+90{ \beta }-25{ \beta }^{ 2 }=56\)
\(19{ \beta }^{ 2 }-90{ \beta }+56=0\)
\(\Rightarrow ({ \beta }-4)(19{ \beta }-14)=0\)
\(\Rightarrow { \beta }=4\)
\({ \beta }=\frac { 14 }{ 19 } \)
When \(\beta\) = 4, the other roots are \(\frac { 3 }{ 2 } (4),4,\frac { 18-5 }{ 2 } (4)\)
\(\Rightarrow 6,4,-1\)

When \(\\ \beta =\frac { 14 }{ 19 } ,\) the other roots are \(\frac { 3 }{ 2 } \beta ,\beta \frac { 18-5\beta }{ 2 } [by(2)]\)
\(\Rightarrow \frac { 3 }{ 2 } \left( \frac { 14 }{ 19 } \right) ,\frac { 14 }{ 19 } ,\frac { 18-5\left( \frac { 14 }{ 19 } \right) }{ 2 } \Rightarrow \frac { 21 }{ 19 } ,\frac { 14 }{ 19 } ,\frac { 136 }{ 19 } \)
3.
Let α, β and γ be the roots of x3+ ax2+ bx + c = 0
Then, we get
Σ1 = α + β + γ = -a ....(1)
Σ2 = αβ + βγ + γα = b ...(2)
Σ3 = αβγ = -c ...(3)
We have to form the equation whose roots are α2, β2 and γ2.
Using (1), (2) and (3), we find the following
Σ1 = α2 + β2 + γ2 = (α + β + γ )2 - 2( αβ + βγ + γα) = (-a)2 -2(b) = a2-2b,
Σ2 = α2β2 + β2γ2 + γ2α2 = (αβ + βγ + γα)2 - 2((αβ)( βγ)(γα) + (γα)(αβ))
= (αβ + βγ + γα)2 - 2αβγ (β + γ + α) = (b)2 - 2(-c)(-a) = b2-2ca
Σ3 = α2β2γ2 = (αβγ)2 = (-c)2 = c2
Hence, the required equation is
x3-(α2 + β2 + γ2)x2 + (α2β2 + β2γ2 + γ2α2)x - α2β2γ2 = 0
That is, x3-(a2-2b)x2 + (b2-2ca)x-c2 = 0
4.
Given cubic equation 3x2-16x2+23x-6 = 0
Let ∝, \(\frac{1}{\alpha}\) and ૪ be the roots of the equation
[∵ product of two roots is 1]
\((1)\rightarrow { x }^{ 2 }-\frac { 16 }{ 3 } { x }^{ 2 }+\frac { 23 }{ 3 } -2=0 \) ......(1)
comparing (1) with
\({ x }^{ 3 }-\left( \frac { \alpha +\beta +\gamma }{ \alpha } \right) +\left( \alpha \frac { 1 }{ \alpha } +\frac { 1 }{ \alpha } .\gamma +\gamma \alpha \right) \)
\(-\alpha \frac { 1 }{ \alpha } .\gamma =0\) ..........(2)
We get,
\(\alpha +\frac { 1 }{ \alpha } +\gamma =\frac { 16 }{ 3 } \) .......(3)
\(1+\frac { \gamma }{ \alpha } +\gamma \alpha =\frac { 23 }{ 3 } \)
\(\alpha .\frac { 1 }{ \alpha } .\gamma =2\Rightarrow \gamma =2\) ............(4)
Substituting ૪ = 2 in (3)
\(\alpha +\frac { 1 }{ \alpha } +2=\frac { 16 }{ 3 } \)
\(\Rightarrow \alpha +\frac { 1 }{ \alpha } =\frac { 16 }{ 3 } -2=\frac { 16-6 }{ 3 } =\frac { 10 }{ 3 } \)
\(\Rightarrow \frac { { \alpha }^{ 2 }+1 }{ \alpha } =\frac { 10 }{ 3 } \)

\({ 3x }^{ 2 }+3=10\alpha \)
\({ 3\alpha }^{ 2 }-10\alpha +3=0\)
\(\alpha =\frac { -10 }{ 3 } or\ \alpha =\frac { 1 }{ 3 } \)
\((3\alpha +10)(3\alpha -1)=0\)
\(\alpha =\frac { -10 }{ 3 } \) is not possible \(\Rightarrow \alpha =\frac { 1 }{ 3 } \)
[\(\because \alpha =\frac { -10 }{ 3 } \) will not satisfy(5)]
∴ The roots are 3, \(\frac{1}{3}\), 2.
5.
Given \((\sqrt { 5 } -\sqrt { 3 } )\) is a root
Another root \(\Rightarrow \sqrt { 5 } +\sqrt { 3 } \)
∴ Sum of the roots \(=\sqrt { 5 } -\sqrt { 3 } +\sqrt { 5 } +\sqrt { 3 } =2\sqrt { 5 } \)
Product of the roots
\(=(\sqrt { 5 } -\sqrt { 3 } )(\sqrt { 5 } +\sqrt { 3 } )\)
\(=(\sqrt { 5 } )^{ 2 }-{ (\sqrt { 3 } ) }^{ 2 }=5-3=2\)
∴ One of the factor is x2 -x (sum of the roots) + product of the roots
\(\Rightarrow { x }^{ 2 }-2x\sqrt { 5 } +2\)
The other factor also will be \({ x }^{ 2 }-2x\sqrt { 5 } +2\)
\(({ x }^{ 2 }-2x\sqrt { 5 } +2)({ x }^{ 2 }+2x\sqrt { 5 } +2)=0\)
\(\Rightarrow ({ x }^{ 2 }+2-2\sqrt { 5 } x)({ x }^{ 2 }+2+2\sqrt { 5 } x)=0\)
\(\Rightarrow \left( { x }^{ 2 }+2 \right) ^{ 2 }-{ (2\sqrt { 5 } x) }^{ 2 }=0\)
\([\because (a+b)(a-b)={ a }^{ 2 }-{ b }^{ 2 }]\)
\(\Rightarrow { x }^{ 4 }+{ 4x }^{ 2 }+4{ (5)x }^{ 2 }=0\)
\(\Rightarrow { x }^{ 4 }+{ 4x }^{ 2 }+4-{ 20x }^{ 2 }=0\)
\(\Rightarrow { x }^{ 4 }-{ 16x }^{ 2 }+4=0\) is a rational co-efficient polynomial equation.
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