# Need help with 2 equations. Simple / Complex Familiar. SOLVE equation (1 Viewer)

#### Abzzski

##### New Member
Need help with solving these 2 questions

Q1. Solve (x + 5)(x - 4) = 0

Q2. Solve x² - 8x + 15 = 0

1. (x+5)(x-4)=0
x^2 -4x+5x-20=0
x^2+x-20=0
x^2+x=20
x^2=20-x
x=4

2. x^2-8x+15=0
x^2+15=8x
x^2=8x-15
x=3

#### fan96

##### 617 pages
1. (x+5)(x-4)=0
x^2 -4x+5x-20=0
x^2+x-20=0
x^2+x=20
x^2=20-x
x=4
It's easier to solve this equation without expanding it.

First note that if two numbers multiply to give zero, then that means at least one of them must be zero.

$\bg_white (x+5)(x-4) = 0$

$\bg_white \begin{cases}x+5=0 \\ x-4=0 \end{cases}$

$\bg_white x = -5 \quad {\rm OR} \quad x = 4$

You've also missed a solution $\bg_white x = 5$ for question 2. I'm not familiar with how Standard Mathematics solves quadratics, but if you were allowed to digitally graph it then it would be easier to find the missing solution.

#### imogens

##### New Member
i dont know if we're allowed to digitally graph, but thanks for the extra solution im such a dummy i was thinking that 5x4 was 80 which threw off my working out lol

#### Drongoski

##### Well-Known Member
1. (x+5)(x-4)=0
x^2 -4x+5x-20=0
x^2+x-20=0
x^2+x=20
x^2=20-x
x=4

2. x^2-8x+15=0
x^2+15=8x
x^2=8x-15
x=3
As fan96 has pointed out, I'm amazed you are doing the questions that way. Luckily for you, you now have feedback on your own effort. Try to improve your approach.

Last edited:

#### JupiterPie

##### New Member
Q1. (x + 5)(x - 4) = 0
Solution:
x + 5 = 0 OR x - 4 = 0
x = -5 OR x = 4

Q1. x² - 8x + 15 = 0
x² - 5x - 3x + 15 = 0
x(x - 5) -3(x - 5) = 0
(x - 5)(x - 3) = 0
(x - 5) = 0 OR (x - 3) = 0
x = 5 OR x = 3