well, to write the definition of "Perpendicular lines form right angel" using "if and only if". the sentence would become : Two lines are perpendicular if and only if they form a right angle Hope this helps
"Two perpendicular lines form right angles." p = two lines are perpendicular q = two lines form right angles
p --> q ≡ IF two lines are perpendicular THEN two lines form right angles.
converse will be: q --> p q -->p ≡ IF two lines form right angles THEN two lines are perpendicular. this is always true. and there is no counterexample to it. therefore q --> p is also always true because this is the definition of two perpendicular lines is fact.
so we can write bi-conditional statement as p <--> q and say two lines are perpendicular IF AND ONLY IF two lines form right angles.
