Use Pythagoras theorem in 3D problems
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 rightangled 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.
AC^{2} = AB^{2} + BD^{2} = 4^{2 }+ 3^{2} = 5^{2}
Now, we have AC and CD, we can calculate AD
AD^{2} = AC^{2} + CD^{2} = 5^{2} + 2^{2} = 29
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