# Math Help - Quadratic Formula Question #4

1. ## Quadratic Formula Question #4

http://i.imgur.com/JBhvQC7h.jpg

I have no clue how I can make a quadratic equation out of this question so I have not attempted anything yet,

any help would be great! I have just started quadratic formula in school, so please be slow.

Thanks.

2. ## Re: Quadratic Formula Question #4

The surface area (s.a) of a cylinder is given by the formula:

$s.a = 2\pi r^2 + 2 \pi rh$

The first part of that formula comes from the surface area of the two circles at the top and bottom of the cylinder, and the second part comes from the perimeter of the cylinder multiplied by its height.

Using the information given in the question:

$h = 5, s.a = 100$

$2\pi r^2 + 10 \pi r = 100$

$r^2 + 5r = \frac{100}{2 \pi}$

$r^2 + 5r - \frac{100}{2 \pi} = 0$

This is a disgusting quadratic equation, but using the quadratic formula will give you a value for r = 2.2cm or r = -7.2cm.

Since r cannot be negative, r must be 2.2cm.

Subbing r = 2.2 back into the original equation should give you a value of approximately 100, meaning the value of r is correct.