Question 259973
You are both wrong!
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Your mistake:
{{{x^2-8x-15=0}}}
x={{{(-(-8)+- sqrt(-8^2-4*1*15))/(2*1)}}} <--Here -- should be -15
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Unless your original equation has a typo then the quadratic equation yields:
x = {9.568, -1.568}
Details of quadratic equation follows:
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*[invoke quadratic "x", 1, -8, -15 ]