Calculus Derivatives
1. Problem: Evaluate $(f \circ g)'(1)$ given $f(1)=2$, $g(1)=2$, $f'(1)=3$, $g'(1)=2$, $f'(2)=1$, and $g'(2)=3$.
Formula: The derivative of a composite function is given by the chain rule:
$$ (f \circ g)'(x) = f'(g(x)) \cdot g'(x) $$
Step 1: Calculate $f'(g(1))$.
Since $g(1) = 2$, we need $f'(2)$ which is given as 1.
Step 2: Calculate $g'(1)$ which is given as 2.
Step 3: Multiply these values:
$$ (f \circ g)'(1) = f'(g(1)) \cdot g'(1) = 1 \times 2 = 2 $$
Answer: (A) 2
2. Problem: An object is dropped and hits the ground after 5 seconds. Find the height from which it was dropped.
Given: acceleration $a(t) = -9.8$ m/s² (taking upward as positive), initial velocity $v_0 = 0$ (dropped), time $t=5$ s.
Formula: Position function under constant acceleration:
$$ s(t) = s_0 + v_0 t + \frac{1}{2} a t^2 $$
Since the object hits the ground at $t=5$, final position $s(5) = 0$.
Step 1: Substitute known values:
$$ 0 = s_0 + 0 \times 5 + \frac{1}{2} (-9.8) \times 5^2 $$
Step 2: Simplify:
$$ 0 = s_0 - 4.9 \times 25 $$
$$ s_0 = 4.9 \times 25 = 122.5 $$
Answer: (E) 4.9 × (25)m
3. Problem: Population growth is proportional to the current population. Initial population $P_0=10000$, doubles in 15 years. Find time to triple.
Formula: Exponential growth:
$$ P(t) = P_0 e^{kt} $$
Step 1: Use doubling info to find $k$:
$$ 2P_0 = P_0 e^{15k} \Rightarrow 2 = e^{15k} \Rightarrow \ln 2 = 15k \Rightarrow k = \frac{\ln 2}{15} $$
Step 2: Find $t$ when population triples:
$$ 3P_0 = P_0 e^{kt} \Rightarrow 3 = e^{kt} \Rightarrow \ln 3 = kt = \frac{\ln 2}{15} t $$
Step 3: Solve for $t$:
$$ t = \frac{15 \ln 3}{\ln 2} $$
Answer: (D) 15 ln(3)/ln(2)
4. Problem: Side of cube increases at 3 cm/s. Find rate of volume increase when side length is 2 cm.
Formula: Volume of cube:
$$ V = s^3 $$
Rate of change:
$$ \frac{dV}{dt} = 3s^2 \frac{ds}{dt} $$
Step 1: Substitute $s=2$ cm and $\frac{ds}{dt} = 3$ cm/s:
$$ \frac{dV}{dt} = 3 \times 2^2 \times 3 = 3 \times 4 \times 3 = 36 $$
Answer: (E) 36 cm³/s
5. Problem: Find minimum slope of tangent to curve $f(x) = x^5 + x^3 - 2x$.
Step 1: Find derivative:
$$ f'(x) = 5x^4 + 3x^2 - 2 $$
Step 2: Find critical points of $f'(x)$ by differentiating again:
$$ f''(x) = 20x^3 + 6x $$
Set $f''(x) = 0$:
$$ 20x^3 + 6x = 0 \Rightarrow x(20x^2 + 6) = 0 $$
Solutions:
$$ x=0 \quad \text{or} \quad 20x^2 + 6=0 \Rightarrow x^2 = -\frac{6}{20} $$
No real roots from second term, so only $x=0$.
Step 3: Check values of $f'(x)$ at critical points and limits:
As $x \to \pm \infty$, $5x^4$ dominates and is positive, so $f'(x) \to +\infty$.
Evaluate $f'(0) = -2$.
Step 4: Since $f'(0) = -2$ is a minimum value of the slope.
Answer: (C) -2