This one is a little bit tricky:

The surface area of a closed cylinder is given by the formula SA = 2(pi)r(r+h) where 'r' centimetres is the radius of the can and 'h' cm is the height. The height of a can of wood finish is 7 cm and its surface area is 231cm^2.

(A) Use the quadratic formula to solve the equation and hence find the radius of the can

Question

This one is a little bit tricky:

The surface area of a closed cylinder is given by the formula SA = 2(pi)r(r+h) where 'r' centimetres is the radius of the can and 'h' cm is the height. The height of a can of wood finish is 7 cm and its surface area is 231cm^2. 

(A) Use the quadratic formula to solve the equation and hence find the radius of the can

directrix · Answer

SA =  2*pi *r * h  +2* pi r^2   

231 = 2 * pi * r * 7  + 2 * pi * r^2

directrix · Answer

231 = 2 * π * r * 7  + 2 *π * r²

 2 *π * r²  + 14 π * r  - 231 = 0

directrix · Answer

r = -10.501 or r = 3.5011

Check this.

directrix · Answer

@Chad123

anonymous · Answer

Your a genius. Thanks for your help

anonymous · Answer

How did you get 3.5?

anonymous · Answer

Could you please show me how? Thx

directrix · Answer

@Chad123 I am not a genius. You can use the quadratic formula to crank out the values of r. Follow this link to see where I did it. http://www.wolframalpha.com/input/?i=+2+*%CF%80+*+r%C2%B2++%2B+14+%CF%80+*+r++-+231+%3D+0

directrix · Answer

Scroll down and click on "approximate form."