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How to find the common root between two quadratic equations?

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roro6992
roro6992 2013-8-23
Hi evryone,
Am a beginner to Matlab,m trying to find out the common root between two quadratic equations of same variable x..plzz tell me how to do it. Plzz help!

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回答（2 个）

Matt J
Matt J 2013-8-23
编辑：Matt J 2013-8-23
The ROOTS command should let you find and compare the roots of the 2 quadratics.
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roro6992
roro6992 2013-8-23
编辑：roro6992 2013-8-23
But in dis case I need to find common root within a function whr I cnnt see d roots n compare by myslf. Is dere any method for directly finding d common root?? Plzz reply!
Matt J
Matt J 2013-8-24
编辑：Matt J 2013-8-24
Using the ROOTS command you can see the roots.

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Walter Roberson
Walter Roberson 2013-8-24
Let the two equations be of the form
a1*x^2+b1*x+c1 = 0
a2*x^2+b2*x+c2 = 0
then if
(1/4)*a2*b1^2/a^2+(-(1/2)*b2/a+(1/2)*a2*(b1^2-4*a1*c1)^(1/2)/a^2)*b1+(1/4)*a2*(b1^2-4*a1*c1)/a^2+c2-(1/2)*b2*(b1^2-4*a1*c1)^(1/2)/a
is 0, then the common root is
(-b1-sqrt(b1^2-4*a1*c1))/(2*a)
and otherwise the common root is
(-b1+sqrt(b1^2-4*a1*c1))/(2*a)
Provided, that is, that there really is a common root.

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