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Given that the zeroes of the cubic polynomial \( x^{3}-6 x^{2}+3 x+10 \) are of the form \( a \), \( a+b, a+2 b \) for some real numbers \( a \) and \( b \), find the values of \( a \) and \( b \) as well as the zeroes of the given polynomial.
Given:
The zeroes of the cubic polynomial \( x^{3}-6 x^{2}+3 x+10 \) are of the form \( a \), \( a+b, a+2 b \) for some real numbers \( a \) and \( b \).
To find:
Here, we have to find the values of \( a \) and \( b \) as well as the zeroes of the given polynomial.
Solution:
Let $f(x)=x^{3}-6 x^{2}+3 x+10$
$a,(a+b)$ and $(a+2 b)$ are the zeroes of $f(x)$.
This implies,
Sum of the zeroes $=-\frac{(\text { Coefficient of } x^{2})}{(\text { Coefficient of } x^{3})}$
Therefore,
$a+(a+b)+(a+2 b)=-\frac{(-6)}{1}$
$3 a+3 b=6$
$a+b=2$.........(i)
Sum of product of two zeroes at a time $=(\frac{\text { Coefficient of } x}{\text { Coefficient of } x^{3}})$
This implies,
$a(a+b)+(a+b)(a+2 b)+a(a+2 b)=\frac{3}{1}$
$a(a+b)+(a+b)\{(a+b)+b\}+a\{(a+b)+b\}=3$
$2 a+2(2+b)+a(2+b)=3$
$2 a+2(2+2-a)+a(2+2-a)=3$
$2 a+8-2 a+4 a-a^{2}=3$
$-a^{2}+8=3-4 a$
$a^{2}-4 a-5=0$
$a^2-5a+a-5 = 0$
$a (a - 5) + 1 (a - 5) = 0$
$(a - 5) (a + 1) = 0$
$a = 5$ or $a=-1$
This implies,
If $a = -1$, then $b = 2-(-1)=2+1=3$ [From (i)]
If $a = 5$, then $b=2-5=-3$ [From (i)]
Therefore,
Required zeroes of $f(x)$ are, When, $a = -1, b = 3$, then
$a,(a+b),(a + 2) = -1, (-1+3), (-1+6)$
$=-1,2, 5$
When, $a = 5, b = -3$, then
$a, (a + b), (a + 2b) = 5, (5 -3), (5 -6)$
$=5,2,-1$
Hence, the required values of $a$ and $b$ are $a = - 1$ and $b= 3$ or $a = 5, b = -3$ and the zeroes are $-1,2$ and $5$.