Question

The roots of the quadratic equation x²+bx+c = 0 are a and ß a) Evaluate i) a² + ß², ii) (a − ß)² b) Find the quadratic equation whose roots are (a² +ß²) and (a - 3)²​

Answer 1

If ɑ and β are the roots of x² + bx + c = 0, then we can write

x² + bx + c = (x - ɑ) (x - β)

Expanding the right side gives

x² + bx + c = x² - (ɑ + β) x + ɑβ

so that ɑ + β = -b and ɑβ = c.

Recall that for all real numbers m and n,

(m + n)² = m² + 2mn + n²

a) It follows that

(i) ɑ² + β² = (ɑ + β)² - 2ɑβ = (-b)² + c = b² + c

(ii) (ɑ - β)² = ɑ² - 2ɑβ + β² = b² + c - 2c = b² - c

b) I assume you mean to find the quadratic whose roots are ɑ² + β² and (ɑ - β)² (and not (ɑ - 3)²). The simplest quadratic of this form is

(x - (ɑ² + β²)) (x - (ɑ - β)²)

Using the results from part (a), this becomes

(x - (b² + c)) (x - (b² - c))

and expanding, we get

x² - (b² + c + b² - c) x + (b² + c) (b² - c)

= x² - 2b² x + b⁴ - c²