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Published on: 02/01/2020
Theory of Equations
Download Tamil Nadu 12th Standard Maths question papers, model tests, one-mark questions, important questions, and public exam papers in PDF format. Free study materials and answer keys for TN State Board students.
Questions + Answers key
Take MCQ Maths Test1.
If the sum of the roots of the quadratic equation ax2+ bx + c = 0 (abc ≠ 0) is equal to the sum of the squares of their reciprocals, then \(\frac { a }{ c } ,\frac { b }{ a } ,\frac { c }{ b } \) are H.P.
2.
Solve the equation (x-2) (x-7) (x-3) (x+2)+19 = 0
3.
If 2+i and 3-\(\sqrt{2}\) are roots of the equation x6-13x5+ 62x4-126x3+ 65x2+127x-140 = 0, find all roots.
4.
Find the sum of squares of roots of the equation 2x4- 8x3+ 6x2-3 = 0.
5.
Find the number of positive integral solutions of (pairs of positive integers satisfying) x2 - y2 = 353702.
6.
If α, β and γ are the roots of the cubic equation x3+ 2x2+ 3x + 4 = 0, form a cubic equation whose roots are \(\frac { 1 }{ \alpha } ,\frac { 1 }{ \beta } ,\frac { 1 }{ \gamma } \)
7.
Find a polynomial equation of minimum degree with rational coefficients, having 2-\(\sqrt{3}\) as a root.
8.
Find the sum of the squares of the roots of ax4+ bx3+ cx2+ dx + e = 0. \(a \neq 0\)
9.
Show that the equation 2x2− 6x +7 = 0 cannot be satisfied by any real values of x.
10.
If the equation ax2+ bx+c = 0(a > 0) has two roots ∝ and β such that ∝ <- 2 and β > 2, then __________
b2-4ac = 0
b2 - 4ac <0
b2 - 4ac >0
b2 - 4ac ≥ 0
11.
The equation \(\sqrt { x+1 } -\sqrt { x-1 } =\sqrt { 4x-1 } \) has ____________
no solution
one solution
two solution
more than one solution
12.
13.
The polynomial x3 - kx2 + 9x has three real zeros if and only if, k satisfies
|k| ≤ 6
k = 0
|k| > 6
|k| ≥ 6
14.
According to the rational root theorem, which number is not possible rational zero of 4x7 + 2x4 - 10x3 - 5?
-1
\(\frac { 5 }{ 4 } \)
\(\frac { 4 }{ 5 } \)
5
15.
If ax + by = 1, cx2 + dy2 = 1 have only one solution, then
(1) \(\frac { { a }^{ 2 } }{ c } +\frac { { b }^{ 2 } }{ d } =1\)
(2) \(x=\frac { a }{ c } \)
(3) \(x=\frac { c }{ a } \)
(4) \(x=\frac { b }{ d } \)
16.
1) \(x+\frac { 1 }{ x } =2\)
2) ax2 + bx + c = 0
3) \(\sqrt { x } +\frac { 1 }{ \sqrt { x } } =4\)
4) \({ ax }^{ 2 }+\frac { b }{ x } +c=0\)
1.
\(\alpha +\beta =\frac { 1 }{ { \alpha }^{ 2 } } +\frac { 1 }{ { \beta }^{ 2 } } =\frac { { \alpha }^{ 2 }+{ \beta }^{ 2 } }{ { \alpha }^{ 2 }{ \beta }^{ 2 } } =\frac { { (\alpha +\beta ) }^{ 2 }-2\alpha \beta }{ { \alpha }^{ 2 }{ \beta }^{ 2 } } \)
\(\Rightarrow \frac { -b }{ a } =\frac { \frac { { b }^{ 2 } }{ { a }^{ 2 } } -2\frac { c }{ a } }{ \frac { { c }^{ 2 } }{ { a }^{ 2 } } } =\frac { { b }^{ 2 }-2ac }{ { c }^{ 2 } } \)
\(\Rightarrow \frac { 2a }{ c } =\frac { { b }^{ 2 } }{ { c }^{ 2 } } +\frac { b }{ a } \)
\(\frac { 2a }{ c } =\frac { { ab }^{ 2 }+{ bc }^{ 2 } }{ { ac }^{ 2 } } \)
\(2{ a }^{ 2 }c={ ab }^{ 2 }+{ b }^{ 2 }\)
\(\frac { 2a }{ b } =\frac { b }{ c } +\frac { c }{ a } \) [Dividing by abc]
\(\Rightarrow \frac { c }{ a } ,\frac { a }{ b } ,\frac { b }{ c } \) are in A.P \(\Rightarrow \frac { a }{ c } ,\frac { b }{ a } ,\frac { c }{ b } \) are in H.P
2.
We can solve this fourth degree equation by rewriting it suitably and adopting a technique of substitution. Rewriting the equation as
(x−2)(x−3)(x−7)(x+2)+19 = 0
the given equation becomes
(x2−5x+6)(x2−5x−14)+19 = 0 .
If we take x2 − 5x as y, then the equation becomes (y+6)(y−14)+19 = 0;
that is, y2-8y-65 = 0
Solving this we get solutions y = 13 and y = −5. Substituting this we get two quadratic equations
x2-5x-13 = 0 and x2-5x+5 = 0
which can be solved by usual techniques. The solutions obtained for these two equations together give solutions as \(\frac { 5\pm \sqrt { 77 } }{ 2 } ,\frac { 5\pm \sqrt { 5 } }{ 2 } \).
3.
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.
4.
Given equation is 2x4- 8x + 6x2- 3 = 0
Here a = 2, b = -8, c = 6, d = 0, e = -3
Let ∝, β, ૪ and \(\delta \) be the roots of equation (1)
Then by Vieta's formula,
\(\sum { _{ 1 }= } \alpha +\beta +\gamma +\delta =\frac { -b }{ a } =\frac { -(-8) }{ 2 } =4\)
\(\sum { _{ 2 } } =\alpha \beta +\alpha \gamma +\alpha \delta +\beta \gamma +\beta \delta +\gamma \delta =\frac { c }{ a } =\frac { 6 }{ 2 } =3\)
\(\sum { _{ 3 } } =\alpha \beta \gamma +\alpha \beta \delta +\alpha \gamma \delta +\beta \gamma \delta =\frac { -d }{ a } =\frac { 0 }{ a } \)
\(\sum { _{ 4 } } =\alpha \beta \gamma \delta =\frac { e }{ a } =\frac { -3 }{ 2 } \)
Now, (a+b+c+d)2 = a2+b2+c2+d2+2(ab+ac+ad+bc+cd)
⇒ n∝2+β2+૪2+\(\delta\)2 = (∝ + β + ૪ + \(\delta\))2-2(\(\alpha \beta +\alpha \gamma +\alpha \delta +\beta \gamma +\beta \delta +\gamma \delta \))
∝2 + β2 + ૪2 = 42-2(3) = 16 - 6 = 10
5.
Since x2- y2 is even, either both x and y are even or both x and y are odd.
In any case both (x + y) and (x - y) are even integers
∴ x2 - y2= (x + y) (x +y) must be divisible by 4 But 4 does not divide 353702
∴ x2 - y2 = 353702 has no positive integral solutions.
6.
The roots of x3+2x2+3x+4 = 0 are ∝, β, ૪
∴ ∝ + β + ૪ = -co-efficient of x2 = -2 .........(1)
∝β + β૪ + ૪∝ = co-effficient of x = 3 ..........(2)
-∝β૪ = +4 ⇒ ∝β૪ = -4 .......(3)
From the cubic equation whose roots are \(\frac { 1 }{ \alpha } ,\frac { 1 }{ \beta } ,\frac { 1 }{ \gamma } \)
\(\frac { 1 }{ \alpha } +\frac { 1 }{ \beta } +\frac { 1 }{ \gamma } =\frac { \beta \gamma +\gamma \alpha +\alpha \beta }{ \alpha \beta \gamma } =\frac { 3 }{ -4 } =\frac { -3 }{ 4 } \)
\(\frac { 1 }{ \alpha \beta } +\frac { 1 }{ \beta \gamma } +\frac { 1 }{ \gamma \alpha } =\frac { \gamma +\alpha +\beta }{ \alpha \beta \gamma } =\frac { -2 }{ -4 } =\frac { 1 }{ 2 } \)
\(\left( \frac { 1 }{ \alpha } \right) \left( \frac { 1 }{ \beta } \right) \left( \frac { 1 }{ \gamma } \right) =\frac { 1 }{ \alpha \beta \gamma } =\frac { 1 }{ -4 } =-\frac { 1 }{ 4 } \)
∴ The required cubic equation is
\({ x }^{ 3 }-\left( \frac { 1 }{ \alpha } +\frac { 1 }{ \beta } +\frac { 1 }{ \gamma } \right) { x }^{ 2 }+\left( \frac { 1 }{ \alpha \beta } +\frac { 1 }{ \beta \gamma } +\frac { 1 }{ \gamma \alpha } \right) x-\left( \frac { 1 }{ \alpha } ,\frac { 1 }{ \beta } ,\frac { 1 }{ \gamma } \right) \)
\(\Rightarrow { x }^{ 3 }+\frac { 3 }{ 4 } { x }^{ 2 }+\frac { 1 }{ 2 } x+\frac { 1 }{ 4 } =0\)
Multiplying by 4 we get,
4x3 + 3x2 + 2x + 1 = 0
7.
Since 2-\(\sqrt{3}\)i is a root and the coefficients are rational numbers, 2+\(\sqrt{3}\)i is also a root. A required polynomial equation is given by
x2 −(Sum of the roots) x + Product of the roots = 0
and hence
x2- 4x +1 = 0 is a required equation.
8.
Let α, β, γ and δ be the roots of ax4+ bx3+ cx2+ dx + e = 0
Σ1 = α + β + γ + δ = -\(\frac { b }{ a } \),
Σ2 = αβ + αγ + αδ + βγ + βδ + γδ = \(\frac { c }{ a } \),
Σ3 = αβγ + αβδ + αγδ + βγδ = -\(\frac { d }{ a } \)
Σ4 = αβγδ =\(\frac { e }{ a } \)
We have to find α2 + β2 + γ2 + δ2
Applying the algebraic identity
(a+b+c+d)2 ≡ a2+b2+c2+d2+2(ab+ac+ad+bc+bd+cd),
we get
α2 + β2 + γ2 + δ2 = (α + β + γ + δ)2-2(αβ + αγ + αδ + βγ + βδ + γδ)
= \(\left( \frac { b }{ a } \right) ^{ 2 }-2\left( \frac { c }{ a } \right) \)
= \(\frac { { b }^{ 2 }-2ac }{ { a }^{ 2 } } \).
9.
Δ = b2- 4ac = -20 < 0. The roots are imaginary numbers.
10.
(c)
b2 - 4ac >0
11.
(a)
no solution
12.
(a)
13.
(d)
|k| ≥ 6
14.
(c)
\(\frac { 4 }{ 5 } \)
15.
\(x=\frac { c }{ a } \)
16.
4) \({ ax }^{ 2 }+\frac { b }{ x } +c=0\)
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