Either increasing the radius or the height of a cylinder by six inches will result in the same volume. The original height of the cylinder is two inches. What is the original radius?
Volume of a Cylinder = π • r2 • h
If you increase the radius by 6 the new volume will be π • (r+6)2 • 2
If you increase the height by 6 the new volume will be π • r2 • (2+6)
Set the new volumes equal and solve for r
π • (r2 + 12r + 36) • 2 = π • r2 • 8
First, let's divide both sides by π (r2 + 12r + 36) • 2 = r2 • 8
Next, let's divide both sides by 2 r2 + 12r + 36 = r2 • 4
Subtract r2 from both sides 12r + 36 = 3r2
Subtract (12r + 36) from both sides 0 = 3r2 – 12r – 36
Switch sides, just cuz I prefer the zero on the right 3r2 – 12r – 36 = 0
Factor the quadratic (3r + 6)(r – 6) = 0 This is the hardest step, IMHO.
Set each factor equal to zero 3r + 6 = 0
r = –2 Discard... can't have negative radius.
r – 6 = 0
r = 6 The original radius was 6. .
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