Originally Posted by

**mr fantastic** Did I not I say

which, I would have thought, provides the necessary justification.

Alternatively, use Pythagoras' Theorem. Alternatively, use $\displaystyle \sin 45^0 = \frac{\text{opposite side}}{\text{hypotenuse}}$ (which you should have realised from what I posted about question 2).

Why are you insisting on wanting to use the cosine rule? For a right angle triangle A = 90 degrees and it's just Pythagoras' Theorem. The cosne rule and the sine rule are used when you have a triangle that is NOT a right-triangle. You should know that.