PreCalc B
posted by Emma on .
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?

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 )