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A perfect cube shaped ice cube melts so that the length of its sides are decreasing at a rate of 2insec2\frac{\text{in}}{\text{sec}}. Assume that the block retains its cube shape as it melts. At what rate is the volume of the ice cube changing when the sides are 7in7\text{in} each?

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Q. A perfect cube shaped ice cube melts so that the length of its sides are decreasing at a rate of 2insec2\frac{\text{in}}{\text{sec}}. Assume that the block retains its cube shape as it melts. At what rate is the volume of the ice cube changing when the sides are 7in7\text{in} each?
  1. Volume Formula: Let's denote the side length of the cube as ss (in inches). The volume VV of a cube is given by the formula V=s3V = s^3. We are given that the side length ss is decreasing at a rate of 22 inches per second, which we can write as dsdt=2\frac{ds}{dt} = -2 in/sec (the negative sign indicates a decrease). We want to find the rate of change of the volume, dVdt\frac{dV}{dt}, when s=7s = 7 inches.
  2. Chain Rule Application: To find dVdt\frac{dV}{dt}, we will use the chain rule from calculus, which allows us to relate the rates of change of different quantities. The chain rule tells us that dVdt=dVdsdsdt\frac{dV}{dt} = \frac{dV}{ds} \cdot \frac{ds}{dt}. We already know dsdt\frac{ds}{dt}, so we need to find dVds\frac{dV}{ds} first.
  3. Finding dVds\frac{dV}{ds}: Differentiating the volume formula V=s3V = s^3 with respect to ss, we get dVds=3s2\frac{dV}{ds} = 3s^2. This derivative represents the rate of change of the volume with respect to the side length.
  4. Substitute s=7s=7: Now we can substitute s=7s = 7 inches into the derivative to find the rate of change of the volume when the side length is 77 inches. So, dVds=3×(7in)2=3×49in2=147in2\frac{dV}{ds} = 3 \times (7 \, \text{in})^2 = 3 \times 49 \, \text{in}^2 = 147 \, \text{in}^2.
  5. Calculate dV/dtdV/dt: We can now use the chain rule to find dV/dtdV/dt. We have dV/dt=(dV/ds)(ds/dt)=147in2/sec(2in/sec)=294in3/secdV/dt = (dV/ds) \cdot (ds/dt) = 147 \, \text{in}^2/\text{sec} \cdot (-2 \, \text{in}/\text{sec}) = -294 \, \text{in}^3/\text{sec}. The negative sign indicates that the volume is decreasing.
  6. Final Result: The rate at which the volume of the ice cube is changing when the sides are 77 inches each is 294-294 cubic inches per second.

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