Step by step
The question
Question: find the two equal sides of an isosceles triangle.
The perimeter is 40 cm.
The base is 12 cm.
1 Use the isosceles fact
Isosceles means two sides are equal.
The small marks show which two.
We call each of them x.
2 Write the perimeter
The perimeter is all the sides added: the way round the edge.
So x + x + 12 = 40.
Careful!
40 take away 12 is both equal sides together.
One side is half of it.
3 Group like terms
Terms are the parts between the + and − signs.
Like terms are the same kind: same letters, same powers.
All the x terms go together.
x + x = 2x.
Now we have 2x + 12 = 40.
We want x by itself.
Two things are stuck to x: times 2 and + 12.
The + 12 is on the outside, so it comes off first.
Then the times 2.
4 Remove the + 12
To remove + 12, we take away 12.
We do it on both sides.
Why both sides?
The = sign means both sides are the same amount.
If we change only one side, they are not the same any more.
+ 12 and − 12 make zero.
Cross them out.
40 − 12 = 28.
Now we have 2x = 28.
5 Remove the 2
2x means 2 times x.
We want just x.
To remove times 2, we divide by 2.
We do it on both sides, so they stay equal.
2 divided by 2 is 1.
So just x is left.
28 divided by 2 is 14.
So x = 14!
So each equal side is 14 cm.
6 Check
Let's check: put x = 14 back and add the three sides.
We get the perimeter, 40 cm.
So x = 14 cm is right!
Let's look back.
First: use the isosceles fact.
Then: write the perimeter.
Then: group like terms.
Then: remove the + 12.
Then: remove the 2.
Last: check.