Gas is escaping from a spherical balloon
at the rate of 2cm3/min. How fast is the
surface area changing when the radius is
12 cm? [V = 43
πr3
] and S = 4πr2
This is another related rates problem. We know the rate of change of the balloon's volume with respect to time (dV/dt), and we want to find the rate of change of the balloon's surface area with respect to time (dS/dt) when the radius r = 12 cm.
We're given that the volume V of a sphere is given by V = 4/3πr³ and the surface area S is given by S = 4πr². Also, we know that dV/dt = -2 cm³/min (the negative sign indicates the volume is decreasing).
Differentiating both V and S with respect to time t gives:
dV/dt = 4πr² * dr/dt (from V = 4/3πr³)
and
dS/dt = 8πr * dr/dt (from S = 4πr²).
We can solve the first equation for dr/dt, the rate of change of the radius with respect to time:
dr/dt = (dV/dt) / (4πr²) = -2 cm³/min / (4π12² cm²) = -2 / (4π144) cm/min ≈ -0.0011 cm/min.
Then substitute this value into the second equation to find dS/dt:
dS/dt = 8πr * dr/dt = 8π*12 cm * (-0.0011 cm/min) = -0.334 cm²/min.
So, when the radius of the balloon is 12 cm, the surface area is decreasing at a rate of approximately 0.334 cm² per minute.
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