for (b), when the say ABD, it is just the name of the triangle. probably the examiner wants you to figure out which angle in triangle ABD is the 90 degree angle. looking at the diagram, it obviously should be angle DAB.

remember Pythagoras theorem can be applied in both directions. ...
if BD^2 is equal to AD^2+AB^2, then angle DAB is a right angle.

Thanks!

Question is asking to show that the triangle ABD is a right-angled one rather than the angle is a right angled one (from the diagram we can tell angle ABD is not the right angle).


Notice that in a) the word 'angle' is present when describing that angle, but in b) the word 'angle' is absent.

The later part of the sentence in b) 'is a right-angled triangle' should intuitively point out that the triangle they're asking for is ABD. Cambridge does not have to explicitly put the word 'triangle' when describing one.


So, either show that the 3 sides of the triangle satisfy Pythagoras' Theorem,

i.e a² + b² = c²

Or show that one of the angles is 90°

The right angle is always directly opposite the hypotenuse.

Students must interprete the questions correctly and accurately