Question 174611
When mineral deposits formed a coating 1mm thick on the inside of a pipe, the area through which the fluid can flow was reduced by 20%. Find the original inside diameter of the pipe.
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Reduced by 20% --> 80% remaining, or 0.8 of the original area.
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{{{0.8pi*r^2=pi(r-1)^2}}}
Divide by pi
{{{0.8r^2 = r^2-2r+1}}}
{{{0.2r^2 - 2r + 1 = 0}}}
{{{r^2 - 10r + 5 = 0}}}
*[invoke solve_quadratic_equation 1,-10,5]
Sub r for x, the solver always uses x.
The second answer is less than 1, so it is extraneous.  Ignore it.
{{{r = 5+2sqrt(5) mms}}}
r = apx 9.472 mms