If the surface area of a cone is given by the formula \(S=\pi\cdot l+\pi\cdot r^2,\) then you have to express l in terms of S and r.
1. Express the whole expression \(\pi\cdot l:\)
\(\pi\cdot l=S-\pi\cdot r^2.\)
2. Divide the previous expression by \(\pi:\)
\(l=\dfrac{S-\pi r^2}{\pi}=\dfrac{S}{\pi}-r^2.\)
3. Since \(\pi\approx 3.14,\) then
\(l=\dfrac{S}{3.14}-r^2.\)
Answer: correct choice is C (here should be S not 5 as written)
