RATIONAL ROOT THEOREM

Example 1 :

State the possible rational roots of f(x) and find all rational roots.

f(x) = 5x³ + 29x² + 19x - 5

Solution :

The constant term is 5 and the leading coefficient is -5.

Factors of -5 : ±1, ±5.

Factors of 5 : ±1, ±5.

So, the possible rational roots are

±1/1, ±1/5, ±5/1, ±5/5

or

±1/1, ±1/5, ±5

Check 1 is rational root to f(x).

f(1) = 5(1)³ + 29(1)² + 19(1) - 5

= 5 + 29 + 19 - 5

= 5 + 29 + 19 - 5

= 48 ≠ 0

Check -1 is rational root to f(x).

f(-1) = 5(-1)³ + 29(-1)² + 19(-1) - 5

= -5 + 29 - 19 - 5

= 0

Check 1/5 is rational root to f(x).

f(1/5) = 5(1/5)³ + 29(1/5)² + 19(1/5) - 5

= 5(1/125) + 29(1/25) + 19/5 - 5

= 1/25 + 29/25 + 19/5 - 5

= 1/25 + 29/25 + 19/5 - 5

= 1/25 + 29/25 + 19/5 - 5

= 0

Check -1/5 is rational root to f(x).

f(1/5) = 5(-1/5)³ + 29(-1/5)² + 19(-1/5) - 5

= 5(-1/125) - 29(1/25) - 19/5 - 5

= -1/25 - 29/25 - 19/5 - 5

= -1/25 + 29/25 + 19/5 - 5

= 1/25 + 29/25 + 19/5 - 5

= 98/25 ≠ 0

Check 5 is rational root to f(x).

f(5) = 5(5)³ + 29(5)² + 19(5) - 5

= 625 + 725 + 95 - 5

= 1440 ≠ 0

Check -5 is rational root to f(x).

f(-5) = 5(-5)³ + 29(-5)² + 19(-5) - 5

= -625 + 725 - 95 - 5

= 0

f(x) becomes zero when

x = -5, -1, 1/5

So, the rational roots of f(x) are -5, -1 and 1/5.

Example 2 :

Use the Rational-Root Theorem to find all rational roots of

f(x) = 4x⁴ + 3x³ + 4x² + 11x + 6

Solution :

The constant term is +6 and the leading coefficient is 4.

Factors of 6 : ±1, ±2, ±3, ±6

Factors of 4 : ±1, ±2, ±4

The possible rational roots are

±1, ±2, ±3, ±6, ±1/2, ±3/2, ±1/4, ±3/4

There are sixteen possible rational roots. It's little hard to check each rational root with f(x) as we have done in the previous problem.

So, we can use the graph of the given function to find the actual rational roots.

The smallest possible rational root is –6 and the largest is 6, so graph f on the interval –6 ≤ x ≤ 6 to see where rational roots might be.

The graph above shows that there are two real roots on the interval –1 ≤ x ≤ 0.

Use a graphing utility to find the x-intercepts of the graph of f(x).

The possible rational roots in the interval –1 ≤ x ≤ 0 are

-1,-3/4, -1/2, -1/4

Test these values with f(x).

f(-1) = 0

f(-3/4) = 0

f(-1/2) ≈ 1.38

f(-1/4) ≈ 3.47

So, –1 and –3/4 are the only rational roots of f(x).

Note :

The Rational-Root Theorem identifies all possible rational roots of a polynomial with integer coefficients. So, any real roots that are not identified must be irrational.

Example 3:

Write out all zeroes of P(x) = 2x⁴ - x³ - 26x² - 11x + 12 then sketch the graph of P(x). Clearly show all x- intercepts and y-intercepts.

Solution :

P(x) = 2x⁴ - x³ - 26x² - 11x + 12

The constant term is +12 and the leading coefficient is 2.

Factors of 12 : ±1, ±2, ±3, ±4, ±6, ±12

Factors of 2 : ±1, ±2

The possible rational roots are

±1, ±2, ±3, ±1/2, ±1/3, ±1/4, ±1/6, ±1/12, ±2/3

-1 is a solution.

-3 is a solution.

2x² - 9x + 4 = 0

(2x - 1)(x - 4) = 0

x = 1/2 and x = 4

So, the roots are -1, -3, 1/2 and 4.

To find y-intercept, put x = 0

P(x) = 2x⁴ - x³ - 26x² - 11x + 12

P(0) = 2(0)⁴ - 0³ - 26(0)² - 11(0) + 12

= 12 is the y-intercept.

Example 4:

List all of the possible rational zeros for the given polynomial. Then use synthetic division to locate one of the zeros. Use the quotient to find the remaining zeros.

p(x) = x³ - 6x² + 11x - 6

Solution :

x = 1

x² - 5x + 6 = 0

x² - 2x - 3x + 6 = 0

x(x - 2) - 3(x - 2) = 0

(x - 2)(x - 3) = 0

x = 2 and x = 3

So, the zeroes are 1, 2 and 3.

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