Step by step
The question
Question: find the value of x² − 4x when x = −1.
x2 − 4x
x = −1
1 Put the numbers in
We put −1 in place of x.
4x means 4 times x.
So we write a times sign.
A negative number goes in brackets, so its minus stays with it.
= (−1)2 − 4 × (−1)
4x = 4 × x
2 Powers
Powers come before minus and times.
Order: ( ) → x² √ → × ÷ → + −
(−1)² means −1 times −1.
Minus times minus is plus.
(−1)2 = (−1) × (−1) = 1
That is 1.
= 1 − 4 × (−1)
3 Times and divide
Times comes before minus.
+ × − = −
Plus times minus is minus.
4 times 1 is 4.
So we get −4.
= 1 − (−4)
4 Plus and minus
Taking away a negative is the same as adding.
− (−) = +
So 1 − (−4) becomes 1 + 4.
= 1 + 4
1 plus 4 is 5.
= 5
So x² − 4x is 5 when x = −1.
Let's look back.
First: put the numbers in.
Then: powers.
Then: times and divide.
Last: plus and minus.
Answer
= 5