Question 1025972
Graphing.
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*[illustration bv2.JPG].
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Completing the square.
{{{x^2-2x-15=0}}}
{{{x^2-2x+1-15=1}}}
{{{(x-1)^2=16}}}
{{{x-1=0 +- 4}}}
{{{x=1 +- 4}}}
{{{x=-3}}} and {{{x=5}}}
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Quadratic equation. 
{{{x = (-b +- sqrt( b^2-4*a*c ))/(2*a) }}} 
{{{x = (-(-2) +- sqrt( (-2)^2-4*(1)*9(-15) ))/(2*1) }}} 
{{{x = (2 +- sqrt( 4+60) ))/(2) }}}
{{{x = (2 +- sqrt( 64 ))/(2) }}}
{{{x = (2 +-8)/(2) }}}
{{{x=1 +- 4}}}
{{{x=-3}}} and {{{x=5}}}

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Completing the square.
Gives an exact answer and also can give max/min values from vertex form.
More information than the other two methods.