For the function solve f(x)

Question

For the function f(x)= 4x + 40/x, solve f(x)= 28

Answer 1

4x + 40/x = 28

 

Multiply through by x:

 

4x(x) + (40/x)(x) = 28(x)

 

4x^2 + 40 = 28x

 

Subtract 28x from both sides:

4x^2 + 40 = 28x

      -28x      -28x

 

4x^2 -28x + 40 = 0

 

Divide through by 4:

 

x^2 - 7x + 10 = 0

 

Factor:

 

(x-5)(x-2) = 0

 

x = {2, 5}

 

Check

4(2) + 40/2 = 28?

 

8 + 20 = 28 ...  Check.

 

 

4(5) + 40/5 = 28?

 

20 + 8 = 28  Check.