Can’t see the error in my logic here: 3d Pythagoras and cosine rule.

[galen burnett]

The question is find angle aMe. I don’t need a cosine rule answer, I know that that does give the correct answer; I just need to know why my own method hasn’t worked, I can’t perceive the flaw in my logic, though there must be one as the correct answer is 60 degrees with the cosine rule whereas
i get 80.

 

[galen burnett]

I think I can see it: I ‘ve wrongly assumed aZm to be a right-angle.

 

[SanjevMathemathician]

I think I can see it: I ‘ve wrongly assumed aZm to be a right-angle.

Yep correct. You have to prove it’s a right angle unless it shows the square right angle mark on it.

 

[galen burnett]

Cheers.