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Question

What is the remainder of function $4a^3 - 12a^2 + 14a - 3$ when it is divided by \(\frac{a-1}{2}\)?

This question was previously asked in
RRB NTPC 2019 CBT 1 Question Paper PDF (31-Jan-2021) (Shift 1)
The correct answer is
$\frac{3}{2}$

Polynomial Remainder Calculation

This problem requires finding the remainder when the polynomial $P(a) = 4a^3 - 12a^2 + 14a - 3$ is divided by a linear expression. We will use the Remainder Theorem.

The divisor is given as $\frac{a-1}{2}$. The Remainder Theorem strictly applies to divisors of the form $(a-c)$. If we interpret $\frac{a-1}{2}$ as the divisor $D(a)$, its root is $a=1$, which leads to a remainder $P(1)=3$. However, since 3 is not an option and $\frac{3}{2}$ is the correct answer, we infer that the intended divisor was $a - \frac{1}{2}$.

Identifying Polynomial and Divisor Root

The polynomial is $P(a) = 4a^3 - 12a^2 + 14a - 3$.

Based on the likely intention due to the answer options, we consider the divisor as $D(a) = a - \frac{1}{2}$.

To find the root of the divisor, set $D(a) = 0$: $a - \frac{1}{2} = 0$ $a = \frac{1}{2}$

Applying the Remainder Theorem

The Remainder Theorem states that when a polynomial $P(a)$ is divided by a linear divisor $a - c$, the remainder $R$ is equal to $P(c)$.

In this case, $c = \frac{1}{2}$. Therefore, the remainder is $R = P(\frac{1}{2})$.

Calculating the Remainder

Substitute $a = \frac{1}{2}$ into the polynomial $P(a)$: $R = P\left(\frac{1}{2}\right) = 4\left(\frac{1}{2}\right)^3 - 12\left(\frac{1}{2}\right)^2 + 14\left(\frac{1}{2}\right) - 3$

Perform the calculations: $R = 4\left(\frac{1}{8}\right) - 12\left(\frac{1}{4}\right) + 14\left(\frac{1}{2}\right) - 3$

$R = \frac{4}{8} - \frac{12}{4} + \frac{14}{2} - 3$

$R = \frac{1}{2} - 3 + 7 - 3$

Combine the terms: $R = \frac{1}{2} + (7 - 3 - 3)$ $R = \frac{1}{2} + 1$

$R = \frac{1}{2} + \frac{2}{2}$

$R = \frac{3}{2}$

The remainder is $\frac{3}{2}$.

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