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Related rate of a melting cube's side length

An ice cube is melting. Its shape remains a cube at all times, and its volume VV is decreasing at a constant rate of 2 cm3/min2 \text{ cm}^3/\text{min}.

Let s(t)s(t) denote the side length of the cube (in cm) at time tt (in minutes), so that the volume is V=s3V = s^3. Given that dVdt=−2 cm3/min\frac{dV}{dt} = -2 \text{ cm}^3/\text{min}, find the rate at which the side length is changing, dsdt\frac{ds}{dt}, at the instant when s=4s = 4 cm.

Hint

Differentiate V=s3V = s^3 implicitly with respect to tt using the chain rule to relate dVdt\frac{dV}{dt} and dsdt\frac{ds}{dt}, then substitute the known values dVdt=−2\frac{dV}{dt} = -2 and s=4s = 4.

Step-by-step solution

Start with the volume formula for a cube, V=s3V = s^3, where both VV and ss are functions of time tt. Differentiate both sides with respect to tt using the chain rule: dVdt=3s2dsdt\frac{dV}{dt} = 3s^2 \frac{ds}{dt}. This equation relates the rate of change of volume to the rate of change of side length at any instant.

Substitute the known values dVdt=−2\frac{dV}{dt} = -2 (volume decreasing) and s=4s = 4 into the equation: −2=3(4)2dsdt=48dsdt-2 = 3(4)^2 \frac{ds}{dt} = 48 \frac{ds}{dt}. Solving for dsdt\frac{ds}{dt} gives dsdt=−248=−124\frac{ds}{dt} = -\frac{2}{48} = -\frac{1}{24} cm/min, meaning the side length is decreasing at a rate of 124\frac{1}{24} cm per minute.

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