The circle is made up of 2 lines, this means that they will equal each other.
x(x + 1) = 10 * 9
~Simplify
x² + x = 90
~Subtract 90 to both sides
x² + x - 90 = 90 - 90
~Simplify
x² + x - 90 = 0
~Factor out
(x - 9)(x + 10) = 0
~Set both factors to equal 0
x - 9 = 0
x + 10 = 0
~Solve for x in [ x - 9 = 0 ]
x - 9 + 9 = 0 + 9
x = 9
~Solve for x in [ x + 10 = 0 ]
x + 10 - 10 = 10 - 10
x = -10
Since -10 is negative, we can't use it.
Therefore, x = 9
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