AP Calculus ABeasymcq1 pt

The rate at which a rumor spreads in a town is proportional to the product of the number of people who have heard it and the number who have not. If the town has NN people and x(t)x(t) have heard the rumor, which differential equation models the spread?

A.dxdt=kx(Nx)\dfrac{dx}{dt} = kx(N-x)
B.dxdt=kx\dfrac{dx}{dt} = kx
C.dxdt=k(Nx)\dfrac{dx}{dt} = k(N-x)
D.dxdt=kx2\dfrac{dx}{dt} = kx^2

Explanation

Core Concept

xx people have heard the rumor and NxN-x have not, so the product of the two groups, scaled by kk, gives the rate.

Correct Answer

Adxdt=kx(Nx)\dfrac{dx}{dt} = kx(N-x)

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