Algebra
Polynomials and Remainder Theorem
GRB_1000_SCQ
Grade Class 12
Question:
The polynomials $P(x) = kx^3 + 3x^2 - 3$ and $Q(x) = 2x^3 - 5x + k$, when divided by $(x-4)$ leave the same remainder, then $k$ is equal to:
Step-by-Step Solution
Key Concept: Remainder Theorem: remainder when $P(x)$ is divided by $(x-a)$ is $P(a)$.
Step 1: Apply the Remainder Theorem to find the remainders.
When a polynomial is divided by $(x-4)$, the remainder equals the value of the polynomial at $x=4$. Therefore, we need to find $P(4)$ and $Q(4)$.
Step 2: Calculate $P(4)$ for the polynomial $P(x) = kx^3 + 3x^2 - 3$.
Substituting $x = 4$ into $P(x)$:
$$P(4) = k(4)^3 + 3(4)^2 - 3$$
$$P(4) = k(64) + 3(16) - 3$$
$$P(4) = 64k + 48 - 3$$
$$P(4) = 64k + 45$$
Step 3: Calculate $Q(4)$ for the polynomial $Q(x) = 2x^3 - 5x + k$.
Substituting $x = 4$ into $Q(x)$:
$$Q(4) = 2(4)^3 - 5(4) + k$$
$$Q(4) = 2(64) - 20 + k$$
$$Q(4) = 128 - 20 + k$$
$$Q(4) = 108 + k$$
Step 4: Set the remainders equal and solve for $k$.
Since both polynomials leave the same remainder when divided by $(x-4)$, we have:
$$P(4) = Q(4)$$
$$64k + 45 = 108 + k$$
Subtracting $k$ from both sides:
$$63k + 45 = 108$$
Subtracting $45$ from both sides:
$$63k = 63$$
Dividing both sides by $63$:
$$k = 1$$
**Final Answer:** The value of $k$ is $\boxed{1}$, which corresponds to **Option 2**.
Correct Answer: 2