Can 10, 10 and 21 make a triangle

Step by step

The question

Question: can sides of 10, 10 and 21 make a triangle?

For a triangle, the two short sides together must be longer than the longest side.

Else they cannot meet.

1 Add the two short sides

The longest side is 21.

We lay it down first.

The two short sides are 10 and 10.

Together: 20.

2 Compare with the longest

Swing them up from the ends.

20 is less than 21: they do not even reach each other.

Careful!

Adding the long side to a short one is always enough.

The test is the two short sides.

So no: they cannot make a triangle.

3 Check

Let's check the other two pairs: 10 plus 21 is 31, and 10 plus 21 is 31.

Both are more than the third side.

So only the short pair decides.

So the answer is right!

Let's look back.

First: add the two short sides.

Then: compare with the longest.

Last: check.