To use the rational root theorem to find all the possible rational roots of the equation 7x3−2x2−14x+4=0 and to find all the actual rational roots if any.
The possible rational roots are 1,−1,2,−2,4,−4,17,−17,27,−27,47 and −47 .
The only rational root of the equation 7x3−2x2−14x+4=0 is 27 .
Given:
The equation 7x3−2x2−14x+4=0 .
Concept Used:
Rational Root Theorem:
Let p(x)=anxn+an−1xn−1+⋅⋅⋅a0 be a polynomial with integer coefficients. There are a limited number of possible roots of p(x)=0 :
- Integer roots must be factors of a0 .
- Rational roots must have reduced form pq where p is an integer factor of a0 and q is an integer factor of an .
Calculation:
Suppose that p(x)=7x3−2x2−14x+4 .
Observe that the leading coefficient of p(x) is 7 and that the constant term of p(x) is 4 .
Now, the leading coefficient 7 has only four factors: 1,−1,7 and −7 .
Also, the constant term 4 has only six factors: 1,−1,2,−2,4 and −4 .
Now, by the rational root theorem, the possible rational roots of p(x) are 11,−11,21,−21,41,−41,1−1,−1−1,2−1,−2−1,4−1,−4−1,17,−17,27,−27,47,−47,1−7,−1−7,2−7,−2−7,4−7 and −4−7 .
That is, the possible rational roots of p(x) are 1,−1,2,−2,4,−4,17,−17,27,−27,47 and −47 .
x | 1 | −1 | 2 | −2 | 4 | −4 | 17 | −17 | 27 | −27 | 47 | −47 |
p(x) | 7⋅13−2⋅12−14⋅1+4=−5 | 9 | 24 | −32 | 364 | −420 | 9749 | 29149 | 0 | 37649 | −16449 | 49249 |
The table shows the values of the function p(x) for the possible roots, 1,−1,2,−2,4,−4,17,−17,27,−27,47 and −47:
Observe that only 27 is an actual root of the polynomial.
Thus, the only rational root of the polynomial p(x)=7x3−2x2−14x+4 is 27 .
That is, the only rational root of the equation 7x3−2x2−14x+4=0 is 27 .
Conclusion:
The only rational root of the equation 7x3−2x2−14x+4=0 is 27 .
Chapter 5 Solutions
High School Math 2015 Common Core Algebra 2 Student Edition Grades 10/11
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