Posted by **Emma** on Friday, January 20, 2012 at 11:22pm.

How do I find zeros on a graph of a cubic function? I can tell that two of the zeros are -4 and 3 because they show on the graph, but how do I find the other?

- Pre-Calc B -
**Anonymous**, Saturday, January 21, 2012 at 1:16am
y = a x ^ 3 + b x ^ 2 + c x + d = a ( x - x1 ) ( x - x2 ) ( x - x3)

y = a [ x - ( - 4 ) ] ( x - 3 ) ( x - x3 )

y = a ( x + 4 ) ( x - 3 ) ( x - x3 )

You must know 4 points on graph to solve this equation.

Maybe your function has a double root (that is, one of the zeroes has a multiplicity of two).

In this case :

y = a ( x + 4 ) ( x + 4 ) ( x - 3 )

OR

y = a ( x + 4 ) ( x - 3 ) ( x - 3 )

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