Working in 3D can be made easy - by working with it in 2D.

Given a 3D shape, you can imagine cutting the shape up to give you a slice, which is flat along where you cut it (and 2D!) 

You need to be able to recognise 2D shapes (flat surfaces) within these 3D shapes. 

For example: [picture missing]

We can still use pythagoras’ theorem to find missing sides and angles, but sometimes there are two steps to the problem.

For example, consider this cuboid.

[pciture missing]

We want to find length AD.

Length AD is also the hypotenuse in the right-angled triangle, with AC and CD as the other 2 sides. 

But first we need to calculate AC because it is not given to us.

AC is also the hypotenuse in another triangle ABD.

AC2 = AB2 + BD2 = 4+ 32 = 52

Now, we have AC and CD, we can calculate AD

AD2 = AC2 + CD2 = 52 + 22 = 29


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