In the study of polynomials, you are aware that a real number ‘k’ is a zero of the polynomial p(x) if p(k) = 0. Remember, zero of a polynomial is different from a zero polynomial. We have seen the Remainder theorem use the concept of zeroes of a polynomial too. In order to understand their importance, we will look at the geometrical representations of linear and quadratic polynomials and the geometrical meaning of their zeroes.
|y = 2x + 3||-1||7|
The straight line y = 2x + 3 will pass through the points (- 2, – 1) and (2, 7). Here is how the graph looks like:
From the Fig.1 above, you can see that the graph of y = 2x + 3 intersects the x-axis at the point (- 3/2, 0). Now, the zero of (2x + 3) is (- 3/2). Therefore, the zero of the linear polynomial (2x + 3) is the x-coordinate of the point where the graph of y = 2x + 3 intersects the x-axis. Hence, we can say,
For a linear polynomial ax + b, a ≠ 0, the graph of y = ax + b is a straight line which intersects the x-axis at exactly one point, namely, (- b/a, 0). Also, this linear polynomial has only one zero which is the x-coordinate of the point where the graph of y = ax + b intersects the x-axis.
Browse more Topics under Polynomials
- Polynomial and its Types
- Value of Polynomial and Division Algorithm
- Degree of Polynomial
- Factorisation of Polynomials
- Remainder Theorem
- Factor Theorem
- Zeroes of Polynomial
Let’s look at a quadratic polynomial, x2 – 3x – 4. To look at the graph of y = x2 – 3x – 4, let’s list some values:
|x||– 2||– 1||0||1||2||3||4||5|
|y = x2 – 3x – 4||6||0||– 4||– 6||– 6||– 4||0||6|
The graph of y = x2 – 3x – 4 will pass through (- 2, 6), (- 1, 0), (0, – 4), (1, – 6), (2, – 6), (3, – 4), (4, 0) and (5, 6). Here is how the graph looks:
For that matter, for any quadratic polynomial y = ax2 + bx + c, a ≠ 0, the graph of y = ax2 + bx + c has either one of these two shapes:
- If a > 0, then it is open upwards like the one shown in the graph above
- If a < 0, then it is open downwards.
These curves are parabolas. A quick look at the table above shows that (-1) and (4) are zeroes of the quadratic polynomial. From the Fig. 2 above, you can see that (-1) and (4) are the x-coordinates of the points where the graph of y = x2 – 3x – 4 intersects the x-axis. Therefore, we can say,
The zeroes of a quadratic polynomial ax2 + bx + c, a ≠ 0, are precisely the x-coordinates of the points where the parabola representing y = ax2 + bx + c intersects the x-axis.
As far as the shape of the graph is concerned, the following three cases are possible:
The graph cuts x-axis at two distinct points A and A′, where the x-coordinates of A and A′ are the two zeroes of the quadratic polynomial ax2 + bx + c, as shown below:
The graph intersects the x-axis at only one point, or at two coincident points. Also, the x-coordinate of A is the only zero for the quadratic polynomial ax2 + bx + c, as shown below:
The graph is either
- Completely above the x-axis or
- Completely below the x-axis.
So, it does not cut the x-axis at any point. Hence, the quadratic polynomial ax2 + bx + c has no zero, as shown below:
To summarize, a quadratic polynomial can have either:
- Two distinct zeroes (as shown in Case i)
- Two equal zeroes (or one zero as shown in Case ii)
- No zero (as shown in Case iii)
It can also be summarized by saying that a polynomial of degree 2 has a maximum of 2 zeroes.
Let’s look at a cubic polynomial, x3 – 4x. Next, let’s list a few values to plot the graph of y = x3 – 4x.
|x||– 2||– 1||0||1||2|
|y = x3 – 4x||0||3||0||– 3||0|
The graph of y = x3 – 4x will pass through (- 2, 0), (- 1, 3), (0, 0), (1, – 3), and (2, 0). Here is how the graph looks like:
From the table above, we can see that 2, 0 and – 2 are the zeroes of the cubic polynomial x3 – 4x. You can also observe that the graph of y = x3 – 4x intersects the x-axis at 2, 0 and – 2. Let’s take a quick look at some examples:
Let’s plot the graph of the following two cubic polynomials:
- x3 – x2
The graphs of y = x3 and y = x3 – x2 look as follows:
From the first graph, you can observe that 0 is the only zero of the polynomial x3, since the graph of y = x3 intersects the x-axis only at 0. Similarly, the polynomial x3 – x2 = x2(x – 1) has two zeroes, 0 and 1. From the second diagram, you can see that the graph of y = x3 – x2 intersects the x-axis at 0 and 1.
Hence, we can conclude that there is a maximum of three zeroes for any cubic polynomial. Or, any polynomial with degree 3 can have maximum 3 zeroes. In general,
Given a polynomial p(x) of degree n, the graph of y = p(x) intersects the x-axis at a maximum of n points. Therefore, a polynomial p(x) of degree n has a maximum of n zeroes.
Solved Examples for You
Answer : The number of zeroes in each of the graphs above are:
- 0, since the line is not intersecting the x-axis.
- 1, since the line intersects the x-axis once.
- 2, since the line intersects the x-axis twice.
- 2, since the line intersects the x-axis twice.
- 4, since the line intersects the x-axis four times.
- 3, since the line intersects the x-axis thrice.
Question 2: What is meant by zeros of the function?
Answer: The zero of a function refers to any replacement for the variable that shall result in an answer of zero. Graphically, the real zero of a function is where the function’s graph crosses the x‐axis. So, the real zero of a function is the x‐intercept(s) of the function’s graph.
Question 3: How can one find zeroes in a function?
Answer: In order to find the zero of a function, one must find the point (a, 0) where the intersection of the function’s graph and the y-intercept takes place. In order to find the value of a from the point (a, 0), one must set the function equal to zero. Afterwards, one must try to solve for x.
Question 4: How many zeroes exist in a linear polynomial?
Answer: A linear polynomial consists of only 1 zero. In contrast, a quadratic polynomial consists of 2 zeroes while a cubic polynomial consists of 3 zeroes.
Question 5: Explain the zeroes in an equation?
Answer: The zeros of a quadratic equation refer to the points where the crossing of the graph of the quadratic equation takes place to the x-axis.