How would you do this? Medal!

Question

pinksapphire · Answer

The Surface area of a cone is... $$21\pi ft^2$$
Slant height is...$$4ft$$
What is the radius?

pinksapphire · Answer

I know the formula for this but how would you solve it? Steps?

poopsiedoodle · Answer

I'd help if I actually knew how to do it. Sorry :<

pinksapphire · Answer

Wait I'll be back...

geerky42 · Answer

Try to arrange formula so the radius is isolated, then plug in value and solve it.

geerky42 · Answer

You have formula for it, right? one with slanted height?

triciaal · Answer

the surface area S = pi *r^2 + pi* r*l where r is the radius and l the slant height

pinksapphire · Answer

That's the one I'm using @triciaal

rishavraj · Answer

u will get two values for radius..ignore the -ve one....simple

pinksapphire · Answer

Can you show me the steps starting at...

$$21\pi=\pi r^2+\pi r l$$

pinksapphire · Answer

What would you do first?

rishavraj · Answer

$$use~\pi*r(l + r) = 21*\pi $$

rishavraj · Answer

cancle pi on both side... and it would be l*r + r^2 = 21...a quadratic

pinksapphire · Answer

Okay...

pinksapphire · Answer

That's how far I got...now I don't know what to do next...

rishavraj · Answer

solve the quadratic.... it would be l = 4
so r^2 + 4r - 21 = 0.... and factorise it

rishavraj · Answer

while factorising it it would be 
r^2 - 3r + 7r - 21

pinksapphire · Answer

Okay Where did the other pi go?

triciaal · Answer

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