Two lines intersect forming four angles

Question

Two lines intersect forming four angles. If one of them is right, show that others are too straight.

I am clueless how to start. Ideas?

when is an angle too straight? – Asinomás May 13 '15 at 03:29 — Asinomás, May 13 '15 at 03:29
It is a right angle, 90 °. – Danilo Santana May 13 '15 at 03:30 — Danilo Santana, May 13 '15 at 03:30
Is it a puzzle? If not try drawing a sketch. – Narasimham May 13 '15 at 03:48 — Narasimham, May 13 '15 at 03:48

score 2 · Answer 1 · edited May 13 '15 at 03:34

2

When two lines intersect, you have that opposite angles are equal, therefore if one is a right angle then the opposite is right too. We also know that two adjacent angles (of this intersection) add up to $180$ degrees, so if one is right, its adjacent one should also be right as to add up to $180$. Finally, the opposite of this one is equally right.

edited May 13 '15 at 03:34

AMPerrine

3,043

answered May 13 '15 at 03:31

Joaquin Liniado

3,840

score 1 · Answer 2 · answered May 13 '15 at 03:37

1

If 2 lines intersect at a right angle, then they are perpendicular. Now, perpendicular lines have all 4 angles being right. So by definition of straight angle, they are straight too.

answered May 13 '15 at 03:37

Teoc

8,700

Two lines intersect forming four angles

2 Answers2