Sign In
If a is a root of P(x)=0, then (x-a) is a factor of P(x). If P(x) is a polynomial with real coefficients, then the complex roots of P(x)=0 occur in conjugate pairs.
P(x)=x^3+3x^2+25x+75
We want to write a polynomial function with rational coefficients so that P(x)=0 has the given roots.
- 3 and 5i
To do so, recall the Conjugate Root Theorem for complex roots.
|
Conjugate Root Theorem |
|
If P(x) is a polynomial with real coefficients, then the complex roots of P(x)=0 occur in conjugate pairs. |
This theorem states that if a + bi is a complex root, then a- bi is also a root. Additionally, recall that if a is a root of P(x)=0, then (x-a) is a factor of P(x).
| Root | Factor |
|---|---|
| 5i | x-5i |
| -5i | x-(- 5i) |
| - 3 | x-(-3) |
| Polynomial | P(x)= (x-5i) (x-(-5i)) (x-(-3)) |
Let's simplify the polynomial by applying the Distributive Property. For simplicity, we will start by multiplying the first two factors together, and then multiply this product by the last factor.
a-(- b)=a+b
Distribute (x+5i)
Distribute x
Distribute -5i
i^2=- 1
- a(- b)=a* b
Subtract terms
Let's continue by multiplying this product by the third factor.