Question

Find two positive numbers whose product is 192 and their sum is a minimum.

Answer 1

Answer:

Since we want a positive number, x = 24.  y = 192/x = 8

Step-by-step explanation:

Let x and y be the two positive numbers.

-  Their product is 192: x*y = 192

-  the sum of the first plus 3 times the second is a minimum:  x + 3y

From the first equation, y = 192/x.  Substitute that into the second equation:

x + 3y = x + 3(192/x) = x + 576/x

Now take the first derivative, set it to zero, and solve for x:

d(x + 576/x)/dx = 1 - 576/x2

0 = 1 - 576/x2

0 = x2 - 576                         [Multiplied both sides by x2]

576 = x2

√576 = √x2

±24 = x

Since we want a positive number, x = 24.  y = 192/x = 8

As a check, the factors of 192 are 192*1, 96*2, 48*4, 24*8, 16*12

192 + 3*1 = 195

96 + 3*2 = 102

48 + 3*4 = 60

24 + 3*8 = 48

16 + 3*12 = 52

24 + 3*8 is indeed the minimum