Question

英文数学题,求高手,急A truck gets 500/x miles per gallon (mpg) when dr

Answer 1

Let z be the total cost (a function of x) and D be the total distance (a constant),then

z = 17 * D/x + 2.8 * D/(500/x)

(The steps above are for explanation purpose only.They are not required.You can alternatively let y be the total cost per unit distance and derive the following.)

Let y (= z/D) = 17/x + 2.8x/500

Now you can minimize y,that is to find a reasonable (x,y) when the derivative of y,y',is 0.

y' = -17x^(-2) + 7/1250 = 0

x^2 = 17*1250/7

x = 55.097 = 55.10