Step by step
The question
Question: find g(−1) for g(x) = x² − 4x.
g(x) = x2 − 4x
g(x) is read "g of x".
It is a rule, called a function.
A number goes in for x.
The rule gives one answer out.
g(x) = "g of x"
x goes in → an answer comes out
1 Put the number in
g(−1) means: put −1 in place of every x.
We write it in brackets.
The brackets keep the minus with the number.
g(−1) = (−1)2 − 4(−1)
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.
4(−1) = 4 × (−1)
+ × − = −
4(−1) means 4 times −1.
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.
g(−1) = 5
So g(−1) = 5.
Let's look back.
First: put the number in.
Then: powers.
Then: times and divide.
Last: plus and minus.
Answer
g(−1) = 5