Step by step
The question
Question: Two positive numbers multiply to 100.
Find the smallest possible sum.
1 Draw and name
Call the first number x, and the second y.
2 Write the sum
The sum is x plus y.
3 Use only one letter
They multiply to 100: x times y is 100.
So y = 100 over x.
Put it in place of y.
4 Which x make sense
The numbers are positive: x is more than 0.
5 Slope = 0
The smallest value is where the slope is 0.
x gives 1.
100/x is 100x to the power −1, which gives −100/x².
Set it to 0, and move the fraction across.
Times both sides by x², so they stay equal.
x squared is 100, so x is 10.
Minus 10 squared is also 100, but x must be positive.
Careful!
The numbers must be positive.
Only x = 10 is allowed.
6 Make sure it is the smallest
Before 10, the slope is negative: going down.
After it, positive: going up.
Down, then up: the bottom.
A minimum.
7 Answer the question
The other number: 100/10 is 10.
The same number!
So the smallest sum is 20.
8 Check
Let's check with x a bit smaller: 9 gives 20.11.
And x = 11 gives 20.09.
Both are bigger than 20.
Right!
Let's look back.
First: draw and name.
Then: write the sum.
Then: use only one letter.
Then: which x make sense.
Then: slope = 0.
Then: make sure it is the smallest.
Then: answer the question.
Last: check.