The student can verify the relationship between zeroes and coefficients for a cubic polynomial.
Introduce the three relationships for a cubic polynomial.
You've mastered quadratic equations, but what happens when the power jumps to 3? A cubic polynomial takes the standard form (where ).
Just as quadratics have up to two zeroes, a cubic polynomial can have up to three zeroes, which we typically call , , and .
The three relationships between zeroes and coefficients for cubics.
Memory Hack:
The signs on the right side alternate: Minus, Plus, Minus . Write them as , , to avoid mixing them up.
Worked example showing how to verify cubic zeroes and relationships.
Problem. Verify that , , and are the zeroes of the cubic polynomial , and then verify the relationship between the zeroes and the coefficients.
Faded example calculating relationships for a simple cubic.
Let us verify the relationship between the zeroes (, , and ) and the coefficients of the cubic polynomial . First, the sum of the zeroes is , which exactly matches . Next, we calculate the sum of the products of the zeroes taken two at a time: ( ) + () + (). This simplifies to , verifying the relationship . Finally, we compute the product of the zeroes as , which corresponds to .
Match the cubic sum/product descriptions to their algebraic formulas.
Terms
Definitions
Stepwise verification of a cubic polynomial's relationships.
Given the cubic polynomial with known zeroes , , and . Verify the relationship for the product of the zeroes.
List the relevant coefficients and zeroes needed for the product.
What is the general formula connecting the product of zeroes to coefficients?
Substitute the numbers into both sides of the relationship.
Show the arithmetic to simplify both sides.
State the final verified equality.