Angles that form a linear pair are those whose addition is 180°. From the figure, it can be seen that <2 plus <3 is equal to 180°.
On the other hand, <4 + <5 = 90°; angles <7 and <9 are equal and its addition would be equal to 180 only if they are equal to 90°, which is not the case; <1 + <2 gives something between 90° and 180°
