Steps:
All cans take on the shape of a cylinder, unless you have seen interesting shape of cans like a starfish.
The formula for surface area of a cylinder is
SA = 2πr2 + 2πrh
where:
r = radius
h = height
Since we know the surface area and height, we can plug them in. Note that we can factor out the 2π. You will see why we factor out 2π rather than 2πr.
2π(r2 + (20)r) = 396
2π(r2 + 20r) = 396
Divide both sides of the equation by 2π to isolate the r terms.
r2 + 20r = 63.025
Subtract 63.025 on both sides of the equation.
r2 + 20r - 63.025 = 0
Use the quadratic formula to solve for r:
r = (-b ± √(b2 - 4ac)) / 2a
where:
a = 1
b = 20
c = -63.025
Plug in these values into the formula. You should get two solutions because of the plus/minus sign. Accept the positive value of r.
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Hope this helps.
