can i please get some help? :)

[BubbleWrapMan]

Let me just invent a tetrahedron

A(0,1,1)
B(0,0,0)
C(-1,0,0)
D(-1,-1,0)

B, C and D are on the plane z = 0 (let's call this the ground)

and , so

But , so so BA is not perpendicular to BD

[Stick]

Interesting to see. However, don't use Methods/Specialist concepts to work out a Further problem because they don't work. My friend found the area of a sector of a circle using a Specialist formula and it did not give him the correct answer (you had to work it out using similarity).

[BubbleWrapMan]

How would that not work? Besides incorrect application

[Stick]

Probably because they just assigned a random angle and radius for the first sector. Anyway, thanks for clearing that up.

[BubbleWrapMan]

In conclusion, it was a bad question

[Jenny_2108]

Let me just invent a tetrahedron

A(0,1,1)
B(0,0,0)
C(-1,0,0)
D(-1,-1,0)

B, C and D are on the plane z = 0 (let's call this the ground)

and , so

But , so so BA is not perpendicular to BD

Okay, if we let B(0,0,0). From the diagram of this question, I can let A(0,0,a), C(c,0,0), D(0,d,0) with a,c,d E R






(same as the question states they are perpendicular)





Thus

You can say they are not true in other situations but in this particular ques, they are true and Stick can use the formula applying for right-angled triangle

[BubbleWrapMan]

What makes it true in this situation? You picked A so it was directly above B, that doesn't mean it is in the question

[Jenny_2108]

What makes it true in this situation? You picked A so it was directly above B, that doesn't mean it is in the question

So what makes it true in your proof related to this question?
You prove that

so BA is not perpendicular to BD

but in question, they already say AB is perpendicular to BD

[BubbleWrapMan]

Oh, I mixed C and D around, let me revise:

Let me just invent a tetrahedron

A(0,1,1)
B(0,0,0)
C(-1,-1,0)
D(-1,0,0)

B, C and D are on the plane z = 0 (let's call this the ground)

and , so

But , so so BA is not perpendicular to BC

[Jenny_2108]

Oh, I mixed C and D around, let me revise:

Let me just invent a tetrahedron

A(0,1,1)
B(0,0,0)
C(-1,-1,0)
D(-1,0,0)

B, C and D are on the plane z = 0 (let's call this the ground)

and , so

But , so so BA is not perpendicular to BC

now, your proof is right but its not related to this ques as the coordinates of A, B, C, D you invent by yourself, not the same with what the diagram depicts

[BubbleWrapMan]

The diagram doesn't depict coordinates

[Jenny_2108]

The diagram doesn't depict coordinates

I don't say your proof wrong but here is what I meant



So

Thus Stick can assume ABC is also a right-angled triangle

[BubbleWrapMan]

My proof was more of a counterexample to show it doesn't have to be a right-angle triangle... I don't know why I'm still talking about this, le sigh

[Jenny_2108]

^ Sr I thought you meant its not a right-angled triangle in this ques 

[BubbleWrapMan]

I was saying that, it doesn't have to be right-angled, though the part that annoys me is we're supposed to assume it is