Why Am I Getting Incorrect Solutions for My Quadratic Equation?

[1/2"]

While solving an problem , i came to this quadratic
x²- 8x +4 =0
I couldn't break up 8x so i to solve it like this,
x= {-(-8)+√(8 ²+ 4x1x4)}/2x1,{-(-8)-√(8 ²+ 4x1x4)}/2x1
but the answers are coming way too absurd and negative.

The actual problem is:
"A hollow right cylinder of uniform thickness is made from the material obtained by melting a solid sphere of copper.If the diameter of the sphere is 12 cm,the height of the cylinder is 72 cm and the external radius of the base of the cylinder is 4 cm then find the thickness of the cylinder to two decimal places. "
And answer according to the book is 0.54 cm( which i am not getting !)


My doing any mistake in calculating or in the formulas?
Please help .I would be extremely grateful!

 

[spamiam]

It's just a silly little arithmetic mistake. The quadratic formula is
$$x = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a}$$

You have pluses inside the radical in both cases.

 

[1/2"]

Yes , i did that .
So what's wrong?

 

[Borek]

You were already told - you have plus inside the radical. Formula calls for minus.

 

[spamiam]

Like I said, you have $$\sqrt{b^2 + 4ac}$$ instead of $$\sqrt{b^2 - 4ac}$$.