Question

What two numbers multiply to 54 and add up to 7

Answer 1

xy = 54
x + y = 7

$xy = 54$
$\frac{xy}{x} = \frac{54}{x}$
$y = \frac{54}{x}$

$x + y = 7$
$x + \frac{54}{x} = 7$
$\frac{x^{2}}{x} + \frac{54}{x} = 7$
$\frac{x^{2} + 54}{x} = 7$
$x^{2} + 54 = 7x$
$x^{2} - 7x + 54 = 0$
$x = \frac{-(-7) \± \sqrt{(-7)^{2} - 4(1)(54)}}{2(1)}$
$x = \frac{7 \± \sqrt{49 - 216}}{2}$
$x = \frac{7 \± \sqrt{-167}}{2}$
$x = \frac{7 \± i\sqrt{167}}{2}$
$x = \frac{7 \± 12.9i}{2}$
$x = 3.5 \± 6.45i$
$x = 3.5 + 6.45i$    $or$    $3.5 - 6.45i$

                  x + y = 7
   3.5 + 6.45i + y = 7
- (3.5 + 6.45i)       - (3.5 + 6.45i)
                        y = 3.5 - 6.45i
                  (x, y) = (3.5 + 6.45i, (3.5 - 6.45i)

                          or

                  x + y = 7
   3.5 - 6.45i + y = 7
- (3.5 - 6.45i)     - (3.5 - 6.45i)
                       y = 3.5 + 6.45i
                 (x, y) = (3.5 - 6.45i, 3.5 + 6.45i)

The two numbers that add up to 7 and can multiply to 54 is 3.5 ± 6.45i.