The question states that the rate of decrease of surface area
A in time
t is constant.
As the rate of change of surface area is constant, this suggests it doesn't depend on
A or
t, so they don't belong in the differential equation.
From this, we can say that, as the rate of decrease of surface area is constant,
dtdA=−kThink of it like a linear graph with a negative gradient. The gradient is constant and negative because it is a whole number and doesn't depend on the
x or
y variable.
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