Question

The government of the Republic of Lemon Island plans to transform their lemon-based economy into a tourism-based economy. They develop the following model to predict the long-term effect of this transformation for the size of their economy y:
dy/dt = (A − Be−t/5)y; A, B > 0.
a. Using A = 0.06 and B = 0.04, find the particular solution y(t) assuming that the size of the economy of Lemon Island before the transformation is $50 000 000, and that the transformation causes the size of the economy to shrink by $10 000 000 at t = 0.
b. The size of the economy of Lemon Island without the transformation can be mod- eled using the same equation with parameters A = 0.02 and B = 0.
In which year after the transformation will the size of the transformed economy surpass the size of the economy without the transformation?
c. Elections on Lemon Island are in five years time. The Lemon Island Polling In- stitute (LIPI) predicts that the current government will only be re-elected, if the growth rate of the economy is, by then, at least 4%. According to the first model, can the current government expect to be re-elected?

Answer 1

Step-by-step explanation:

dy/dt = (A − Be^(-t/5)) y

(a) First, find the general solution by separating the variables and integrating.

dy / y = (A − Be^(-t/5)) dt

dy / y = [A + 5B (-⅕ e^(-t/5))] dt

ln |y| = At + 5B e^(-t/5) + C

y = e^(At + 5B e^(-t/5) + C)

y = Ce^(At + 5B e^(-t/5))

Given that A = 0.06, B = 0.04, and y(0) = 50×10⁶ − 10×10⁶ = 40×10⁶:

40×10⁶ = Ce^(0.06(0) + 5(0.04) e^(-0/5))

40×10⁶ = Ce^(0.2)

C = 40×10⁶ e^(-0.2)

y = 40×10⁶ e^(-0.2) e^(0.06t + 0.2 e^(-t/5))

y = 40×10⁶ e^(-0.2 + 0.06t + 0.2 e^(-t/5))

(b) If A = 0.02 and B = 0, and there is no transformation (y(0) = 50×10⁶), then:

50×10⁶ = Ce^(0.02(0) + 0)

50×10⁶ = C

y = 50×10⁶ e^(0.02t)

Comparing to the answer from part (a):

40×10⁶ e^(-0.2 + 0.06t + 0.2 e^(-t/5)) = 50×10⁶ e^(0.02t)

e^(-0.2 + 0.04t + 0.2 e^(-t/5)) = 5/4

-0.2 + 0.04t + 0.2 e^(-t/5) = ln(5/4)

-5 + t + 5e^(-t/5) = 25 ln(5/4)

t + 5e^(-t/5) = 5 + 25 ln(5/4)

Solve with a calculator:

t = 9.886

The transformed economy surpasses the untransformed economy in the 10th year.

(c) In year t=5, the size of the transformed economy is:

y = 40×10⁶ e^(-0.2 + 0.06(5) + 0.2 e^(-5/5))

y = 47.6×10⁶

The percent growth is:

(47.6×10⁶ − 40×10⁶) / 40×10⁶ × 100% = 19%

The growth rate is greater than 4%, so the current government can expect to be reelected.