Finding Zeroes of a polynomial
The below given fact will also be useful on occasion in determining the zeroes of a polynomial.
Fact
If P (x) is a polynomial & we know that P (a) = 0 and P (b) = 0 then somewhere among a and b is a zero of P ( x ) .
According to this fact if we evaluate the polynomial at two points & one of the evaluations provides a positive value (that means the point is above the x-axis) & the other evaluation provides a negative value (that means the point is below the x-axis), then the single way to get from one point to the other is to go through the x-axis. Or, in other terms, the polynomial ought to have a zero, as we know that zeroes are where a graph touches or crosses the x-axis.
Notice that this fact doesn't tell us what the zero is, it just tells us that one will present. Also, note that if both of the evaluations are +ve or both evaluations are -ve there may or may not be a zero among them.