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Q2(i):
Use the Factor Theorem to determine whether $g(x)$ is a factor of $p(x)$ in each of the following cases: (i) $p(x) = 2x^3 + x^2 – 2x – 1, g(x) = x + 1$

Solution :

Initial Setup & Theoretical Foundation

We are given two polynomials:

  • The dividend polynomial: $p(x) = 2x^3 + x^2 - 2x - 1$
  • The divisor polynomial: $g(x) = x + 1$

To determine whether $g(x)$ is a factor of $p(x)$, we utilize the Factor Theorem. [Per the Factor Theorem, a linear polynomial $x - c$ is a factor of a polynomial $p(x)$ if and only if the polynomial evaluated at $c$ equals zero, i.e., $p(c) = 0$.]

Step 1: Determining the Zero of the Divisor Polynomial $g(x)$

First, we must find the root (or zero) of the linear divisor $g(x)$. We do this by setting the polynomial equal to zero and solving for $x$:

$g(x) = 0$
$x + 1 = 0$
$x = -1$

Thus, the value to be substituted into the dividend polynomial $p(x)$ is $c = -1$.

Step 2: Evaluating the Polynomial $p(x)$ at $x = -1$

We now substitute $x = -1$ into the polynomial $p(x)$ to find the remainder. [By the Remainder Theorem, evaluating $p(-1)$ yields the exact remainder of the division of $p(x)$ by $x + 1$.]

$p(-1) = 2(-1)^3 + (-1)^2 - 2(-1) - 1$

Evaluating each term sequentially based on the order of operations (exponentiation first):

  • $(-1)^3 = -1 \implies 2(-1)^3 = 2(-1) = -2$
  • $(-1)^2 = 1$
  • $-2(-1) = 2$
  • The constant term remains $-1$

Substituting these evaluated terms back into the equation:

$p(-1) = -2 + 1 + 2 - 1$

Grouping the positive and negative terms to simplify the arithmetic:

$p(-1) = (-2 + 2) + (1 - 1)$
$p(-1) = 0 + 0$
$p(-1) = 0$

Step 3: Applying the Factor Theorem

The evaluation yields $p(-1) = 0$. Because the remainder is exactly zero, the condition of the Factor Theorem is perfectly satisfied. This proves that there is no remainder when $2x^3 + x^2 - 2x - 1$ is divided by $x + 1$.

Input Root x = -1 Polynomial p(x) 2x³ + x² - 2x - 1 Remainder p(-1) = 0 Conclusion: g(x) is a FACTOR

Final Solution: Since $p(-1) = 0$, by the Factor Theorem, $g(x) = x + 1$ is a factor of the polynomial $p(x) = 2x^3 + x^2 - 2x - 1$.


More Questions from Class 9 Mathematics Polynomials EXERCISE 2.3


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