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Find two positive numbers whose product is 300 and such that the sum of the first and four times the second is a minimum.

Respuesta :

The two positive numbers whose product is 300 and such that the sum of the first and four times the second is a minimum are 7.66 and 39.16

Let the two variables be x and y

If the product of the numbers is 300, hence

xy = 300

x = 300/y ........ 1

If the sum of the first and four times the second is a minimum, then;

P(x, y) = x + 4y .................. 2

Substitute equation 1 into equation 2:

P(x, y) = x + 4y

P(y) = 300/y + 4y

If this function is at mimimum, hence dP/dy = 0

dP/dy = -300/y² + 4 = 0

-300/y² + 4 = 0

-300 + 4y² = 0

4y² = 300

y² = 300/4

y²=   75

y = 7.66

Since xy = 300

x = 300/7.66

x = 39.16

Hence the two positive numbers whose product is 300 and such that the sum of the first and four times the second is a minimum are 7.66 and 39.16

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