The radius of the base of a cone is increasing at the rate of 3 cm/minute and the altitude is decreasing at th

3.2 · Q6

Q. The radius of the base of a cone is increasing at the rate of 3 cm/minute and the altitude is decreasing at the rate of 4 cm/minute. Find the rate of change of lateral surface area when the radius is 7 cm and the altitude 24 cm.

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Concept understanding — Related Rates

Related Rates: When Two Things Change Together

Imagine you're blowing up a balloon. Your lungs push air in at a certain rate — say, 3 cubic centimetres per second. As the balloon grows, its radius increases. The question is: how fast is the radius increasing at the exact moment when the radius is 5 cm?

That's a related rates problem. Two quantities — the volume $V$ and the radius $r$ — are linked by a geometric formula ( $V = \frac{4}{3}\pi r^3$ ). You know how fast one is changing ( $dV/dt = 3$ ), and you want to find how fast the other is changing ( $dr/dt$ ) at a specific instant.

The core idea is simple: if two quantities are connected by an equation, their rates of change are also connected — by the derivative of that equation.

The Precise Statement

Let $x$ and $y$ be two quantities that both depend on time $t$ , and suppose they satisfy some equation $F(x, y) = 0$ (or $y = f(x)$ , etc.). Then:

Differentiate both sides of the equation with respect to time $t$ .
Use the chain rule wherever you see a variable that depends on $t$ .
The result is an equation linking $dx/dt$ and $dy/dt$ .

That's it. The "related rates" are $dx/dt$ and $dy/dt$ , and the chain rule is the tool that connects them.

\frac{d}{dt} \big[ \text{equation linking variables} \big] \quad\Longrightarrow\quad \text{equation linking rates}

The Chain Rule in Action

In the balloon example, the equation is $V = \frac{4}{3}\pi r^3$ . Differentiate both sides with respect to $t$ :

\frac{dV}{dt} = \frac{d}{dt}\left( \frac{4}{3}\pi r^3 \right)

The right side is a function of $r$ , and $r$ itself depends on $t$ . By the chain rule:

\frac{dV}{dt} = \frac{4}{3}\pi \cdot 3r^2 \cdot \frac{dr}{dt} = 4\pi r^2 \frac{dr}{dt}

Now plug in what you know: $dV/dt = 3$ and $r = 5$ :

3 = 4\pi (5)^2 \frac{dr}{dt} \quad\Longrightarrow\quad 3 = 100\pi \frac{dr}{dt}

So:

\frac{dr}{dt} = \frac{3}{100\pi} \text{ cm/s}

That's the answer. The radius is growing at about 0.0095 cm/s when the balloon's radius is 5 cm.

Watch out

A common mistake is to plug in known values before differentiating. Don't. If you substitute $r = 5$ into $V = \frac{4}{3}\pi r^3$ first, you get a constant — and its derivative is zero. You lose the relationship between the rates. Always differentiate first, then substitute.

The General Recipe

For any related rates problem, follow these steps:

Identify all changing quantities and assign them variables. Note which rates you know and which you need.
Write an equation that relates the quantities at any time $t$ (not just at the instant of interest).
Differentiate implicitly with respect to $t$ . Use the chain rule for every variable that depends on $t$ .
Substitute the known values (including the specific instant's measurements) into the rate equation.
Solve for the unknown rate.

A Second Example: The Ladder Problem …

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With slant height $l=\sqrt{r^2+h^2}=25$ at $r=7,h=24$ , differentiating $S=\pi r l$ gives $\frac{dS}{dt}=54\pi\approx169.6\text{ cm}^2$ /min.

S=\pi r l

Slant height at the instant $r=7,\ h=24$ :

l=\sqrt{7^2+24^2}=\sqrt{49+576}=\sqrt{625}=25\text{ cm}.

Write $S$ in terms of $r,h$ : $S=\pi r\sqrt{r^2+h^2}.$
Differentiate w.r.t. time (product + chain rule):

\frac{dS}{dt}=\pi\left[\frac{dr}{dt}\sqrt{r^2+h^2}+r\cdot\frac{r\frac{dr}{dt}+h\frac{dh}{dt}}{\sqrt{r^2+h^2}}\right].

Compute the inner term $r\frac{dr}{dt}+h\frac{dh}{dt}$ : …

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Q.The radius of the base of a cone is increasing at the rate of 3 cm/minute and the altitude is decreasing at the rate of 4 cm/minute. Find the rate of change of lateral surface area when the radius is 7 cm and the altitude 24 cm.

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