0

Sun 11 Dec, 2011 12:24 am
The Quadratic formula should give x=-8 and x=2, but I keep getting x=8 and x=-2.
Original Equation: x^2 - 6x - 16
Let: a = 1, b = -6, c = -16
x = (-(-6) +- sqrt((-6)^2 - 4(1)(-16)))/2(1)
x = (6 +- sqrt(36 - (-64)))/2
x = (6 +- sqrt(36 + 64))/2
x = (6 +- sqrt(100))/2
x = (6 + 10)/2 = 16/2 = 8
x = (6 - 10)/2 = -4/2 = -2

This looks fine until you plug the numbers in:
(x + 8)(x - 2) = x^2 + 6x - 16
Notice the sign of 6x is different from the original equation. Why?
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fresco

3
Sun 11 Dec, 2011 01:25 am
@etrek,
....because if the roots of a quadratic are p and q, then (x-p)(x-q)=0
NOT (x+p)(x+q)=0
etrek

1
Sun 11 Dec, 2011 02:19 am
@fresco,
Oops, my bad. It's been a while since I studied algebra and it's late (tired). Thanks for the refresh.
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