Derivative Rules of Constant, Sum, Difference
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AP Calculus AB › Derivative Rules of Constant, Sum, Difference
Let $$f(x) = \frac{x^4}{4} - \frac{x^2}{2} + 5$$. For which value of $$x$$ does $$f'(x) = 0$$?
$$x = 1$$
$$x = 2$$
$$x = 5$$
$$x = 4$$
Explanation
First, find the derivative of $$f(x)$$: $$f'(x) = \frac{1}{4}(4x^3) - \frac{1}{2}(2x) + 0 = x^3 - x$$. Now, set the derivative equal to zero: $$x^3 - x = 0$$. Factor out an $$x$$: $$x(x^2 - 1) = 0$$. This gives solutions $$x=0$$ and $$x^2=1$$, so $$x = -1, 0, 1$$. From the given choices, $$x=1$$ is a correct answer.
If $$f(x) = 5x^3 - 2x^2 + x - 9$$, what is $$f'(x)$$?
$$15x^2 - 4x + 1$$
$$15x^2 - 4x$$
$$5x^2 - 2x + 1$$
$$15x^2 - 4x + 1 - 9$$
Explanation
To find the derivative of $$f(x)$$, we apply the power rule, sum/difference rule, and constant rule to each term. The derivative of $$5x^3$$ is $$15x^2$$. The derivative of $$-2x^2$$ is $$-4x$$. The derivative of $$x$$ is $$1$$. The derivative of the constant $$-9$$ is $$0$$. Combining these gives $$f'(x) = 15x^2 - 4x + 1$$.
The position of a particle moving along the x-axis is given by $$p(t) = t^3 - 6t^2 + 5$$ for time $$t \ge 0$$. What is the velocity of the particle at the instant its acceleration is zero?
$$-11$$
$$0$$
$$-12$$
$$2$$
Explanation
The velocity function is the first derivative of the position function: $$v(t) = p'(t) = 3t^2 - 12t$$. The acceleration function is the second derivative of the position function: $$a(t) = p''(t) = 6t - 12$$. To find when the acceleration is zero, set $$a(t) = 0$$: $$6t - 12 = 0$$, which gives $$t = 2$$. To find the velocity at this time, substitute $$t=2$$ into the velocity function: $$v(2) = 3(2)^2 - 12(2) = 3(4) - 24 = 12 - 24 = -12$$.
What is the derivative of $$f(x) = (2x - 3)^2$$?
$$4x - 6$$
$$8x$$
$$2(2x - 3)$$
$$8x - 12$$
Explanation
First, expand the expression for $$f(x)$$: $$f(x) = (2x - 3)(2x - 3) = 4x^2 - 6x - 6x + 9 = 4x^2 - 12x + 9$$. Now, differentiate the expanded polynomial term by term: $$f'(x) = rac{d}{dx}(4x^2 - 12x + 9) = 8x - 12$$.
Let $$f$$ and $$g$$ be differentiable functions. If $$h(x) = 4f(x) - 3g(x) + 2$$, and it is known that $$f'(2) = 5$$ and $$g'(2) = -1$$, what is the value of $$h'(2)$$?
$$25$$
$$23$$
$$14$$
$$17$$
Explanation
First, find the derivative of $$h(x)$$ using the sum, difference, and constant multiple rules. $$h'(x) = 4f'(x) - 3g'(x) + 0$$. Now, substitute $$x=2$$ into the expression for $$h'(x)$$: $$h'(2) = 4f'(2) - 3g'(2)$$. Using the given values, $$h'(2) = 4(5) - 3(-1) = 20 + 3 = 23$$.
Let $f(t)=9t^3-2t^2-6t+1$. What is $f'(t)$?
$27t^2-4t-6$
$9t^2-2t-6$
$9t^3-2t^2-6t+1$
$27t^3-4t^2-6t$
$27t^2-4t-5$
Explanation
Linearity rules allow us to handle the sum and differences in f(t) by differentiating individually with the constant multiple rule. For f(t) = $9t^3$ - $2t^2$ - 6t + 1, apply the power rule to every term. The derivative of $9t^3$ is $27t^2$, of $-2t^2$ is -4t, of -6t is -6, and of 1 is 0. Therefore, f'(t) = $27t^2$ - 4t - 6, corresponding to choice A. A common rule misuse is applying the power rule without multiplying by the exponent, e.g., $t^3$ to $t^2$ instead of $3t^2$. Another is ignoring negative signs in differences. A transferable strategy is to process each term with d/dx (c $x^n$) = c n $x^{n-1}$, remember constants vanish, and maintain sign integrity in the final expression.
Let $F(x)=3x^3-4x^2+5x-6$. What is the derivative $F'(x)$?
$3x^3-4x^2+5x-6$
$9x^2-8x-6$
$9x^3-8x^2+5x$
$3x^2-8x+5$
$9x^2-8x+5$
Explanation
Linearity rules allow independent differentiation of terms in F(x) = $3x^3$ - $4x^2$ + 5x - 6. Derivative of $3x^3$ is $9x^2$, of $-4x^2$ is -8x, of 5x is 5, of -6 is 0. Thus, F'(x) = $9x^2$ - 8x + 5. A common misuse is subtracting instead of adding derivatives in sums. Constants are often incorrectly included. Decompose using linearity and apply power rule to each term for accuracy.
Let $G(x)= -6x^2+13x-15$. What is the derivative $G'(x)$?
$-12x^2+13x$
$-12x+13$
$-6x+13$
$-6x^2+13x-15$
$-12x-15$
Explanation
The sum and difference rules of linearity allow term-by-term differentiation of polynomials. For G(x) = $-6x^2$ + 13x - 15, derivative of $-6x^2$ is -12x, of 13x is 13, and of -15 is 0. Thus, G'(x) = -12x + 13. A common misuse is including the constant's derivative as nonzero, like adding -15. Signs on linear terms must be preserved accurately. For quadratic functions, use linearity to differentiate each power separately, ensuring constants are ignored.
A function is $y(t)=13t^2+4t+9$. What is $y'(t)$?
$26t+9$
$13t+4$
$26t+4$
$26t^2+4t$
$13t^2+4t+9$
Explanation
Linearity rules facilitate term-by-term differentiation for y(t) = $13t^2$ + 4t + 9. Derivative of $13t^2$ is 26t, of 4t is 4, of +9 is 0. Thus, y'(t) = 26t + 4. A common misuse is including the constant in the derivative, like adding 9. Linear terms are sometimes overlooked. For quadratics, apply sum rules and power rule individually to each term.
Let $N(x)=5x^4+7x^3-14x+9$. What is $N'(x)$?
$5x^4+7x^3-14x+9$
$5x^3+21x^2-14$
$20x^3+21x^2+9$
$20x^4+21x^3-14x$
$20x^3+21x^2-14$
Explanation
Linearity rules facilitate term-by-term differentiation for N(x) with sums and multiples. For N(x) = $5x^4$ + $7x^3$ - 14x + 9, use the power rule. The derivative of $5x^4$ is $20x^3$, of $7x^3$ is $21x^2$, of -14x is -14, and of 9 is 0. Therefore, N'(x) = $20x^3$ + $21x^2$ - 14, which is choice A. A common misuse is forgetting multiplication for cubic terms. Some include constants. A transferable strategy is to sequence terms, apply d/dx (c $x^n$) = c n $x^{n-1}$, and combine using linearity.