Vachmi
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+x2−5x+2;12,1,−2
(ii) x3−4x2+5x−2;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+x2−5x+2
p(12)=2×(12)3+(12)2−5(12)+2
=14+14−52+2=0
p(1)=2×13+12−5(1)+2
= 2+1−5+2 = 0
p(−2)=2(−2)3+(−2)2−5(−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.
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
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,