Right angle at A, AD ⊥ BC, AC = 6, BC = 9, find DC

Step by step

The question

Question: triangle A B C has a right angle at A, and A D is at 90° to B C, with AC = 6, BC = 9: find DC.

1 Find the similar triangles

The height A D cuts the big triangle into two small ones.

All three triangles have a right angle, and they share their other angles.

So they are similar: the same shape.

2 Write the rule

Triangles D A C and A B C: D C over A C equals A C over B C.

Cross-multiplied, that gives the rule: AC squared is DC times BC.

Careful!

It is DC times BC, not plus.

3 Put the numbers in

Put the numbers in.

We call DC x.

6 squared is 36.

4 Remove the 9

To remove times 9, we divide both sides by 9: 4.

5 Check

Let's check with Pythagoras in the small triangle A B D: A D squared plus B D squared is 45 the same as A B squared.

So it fits!

Let's look back.

First: find the similar triangles.

Then: write the rule.

Then: put the numbers in.

Then: remove the 9.

Last: check.