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Vachmi

1
Verify that the numbers given alongside of the cubic polynomials below are their zeroes. Also, verify the relationship between the zeroes and the coefficients in each case:
(i) 2x3+x25x+2;12,1,2
(ii) x34x2+5x2;2,1,1


Solution

(i) On comparing the given polynomial with the polynomial ax3+bx2+cx+d,
we obtain a = 2, b = 1, c = -5, d = 2
∴ p(x)= 2x3+x25x+2
p(12)=2×(12)3+(12)25(12)+2 =14+1452+2=0

p(1)=2×13+125(1)+2 = 2+15+2 = 0

p(2)=2(2)3+(2)25(2)+2 =16+4+10+2=0

Hence 12, 1 and -2 are the zeroes.

α = 12, β = 1, γ= -2
α+β+γ = 12 = ba
αβ+βγ+γα = 52 = ca
αβγ = 22 = da
Thus, the relationship between the zeroes and the coefficients is verified.

(ii) On comparing the given polynomial with the polynomial ax3+bx2+cx+d,
we obtain a = 1, b = -4, c = 5, d = -2.
∴ p(x)= x^{3}-4x^{2}+5x-2
p(2)= 2^{3}-4(2)^{2}+5(2)-2 =8-16+10-2=0

p(1)= 1^{3}-4(1)^{2}+5(1)-2= 0

Hence 2, 1 and 1 are the zeroes.
α= 2 β = 1 γ= 1
α+β+γ = \dfrac{4}{1}=\dfrac{-b}{a}

αβ+βγ+γα = \dfrac{5}{1}= \dfrac{c}{a}

αβγ = \dfrac{2}{1}= -\dfrac{d}{a}

Thus, the relationship between the zeroes and the coefficients is verified.




2
Find a cubic polynomial with the sum, sum of the product of its zeroes taken two at a time, and the product of its zeroes as 2, -7, -14 respectively.


Solution

Let the zeroes of the required polynomial be α, β, γ.
∴ α+β+γ = 2 and
αβ+βγ+γα = -7 and
αβγ = -14

The cubic polynomial is of the form
x^3 - (sum of the zeros) x^2 + (sum of the product of the zeros taken two at a time) - (product of the zeros) = 0
⇒ x^3 - (α+β+γ)x^2 + (αβ+βγ+γα)x - (αβγ) = 0
⇒ x^3 -2x^2-7x+14=0 is the required polynomial.




3
If the zeroes of the polynomial x^3-3x^2 + x + 1 are a-b, a, a + b, find a and b.


Solution

Let α, β, and γ be the zeroes of equation p(x) = x^3 - 3x^2 + x + 1
The zeroes of the polynomial p(x) are given as a - b, a, a + b.
∴ Sum of the zeroes = α+β+γ = 3=(a-b)+a+(a+b)
∴ 3a=3 or a=1............(i)
Product of the zeroes αβγ = -1=(a-b)a(a+b)
a^{3}-ab^{2}=-1.........(ii)
Substituting the value of a from (i) and (ii), we get, 1^{3}-1b^{2}=-1
b^{2}=2 or b = \sqrt{2}
Hence a=1 and b=\sqrt{2}




4
If two zeroes of the polynomial x^4 - 6x^3 - 26x^2 + 138x - 35 are 2 ± \sqrt{3}, find other zeroes.


Solution

The 2 zeroes which are given are 2+ \sqrt{3} and 2-\sqrt{3}
∴ \left (x-(2+\sqrt{3}) \right) \left(x-(2-\sqrt{3}) \right)
= (x-2)^{2}-(\sqrt{3})^{2}
Therefore, x^2 - 4x + 1 is a factor of the given polynomial.

Polynomial long division

So x^4 - 6x^3 - 26x^2 + 138x - 35
= (x^{2}-4x+1)(x^2-2x-35)
= (x^{2}-4x+1)(x^2-7x+5x-35)
= (x^{2}-4x+1)(x(x-7)+5(x-7))
= (x^{2}-4x+1)(x-7)(x+5)
Hence the other zeroes of the polynomial are 7 and -5




5
If the polynomial x^4 - 6x^3 + 16x^2 - 25x + 10 is divided by another polynomial x^2 - 2x + k, the remainder comes out to be x + a, find k and a.


Solution

p(x) = x^4 - 6x^3 + 16x^2 - 25x + 10
Dividing by x^2 - 2x + k,


Polynomial long division

As remainder is given as x + a
⇒ (-9+2k)x + 10-8k +k^2 = x + a

On comparing the like coefficients, we get:
-9 + 2k = 1
⇒ k = 5

10-8k+k^2 = a
⇒ 10-8×5 - 25 = a
⇒ 10-8×5 + 25 = a
⇒ a = -5

Hence k = 5 and a = -5