the intersection of two planes is one line true or false?

Answer 1

Sometime true (so false), the intersection could be empty (if the planes are parallel, or one plane (if the planes are the same).

Answer 2

that statement is true if altered slightly...

the intersection of two non-parallel planes is a line.

the intersection of two coincidental planes is just the plane itself, and the intersection of two parallel planes is empty. however, a coincidental plane is obviously parallel to itself, so it remains sufficient and necessary that two planes be non-parallel in order for their intersection to be a line.

another way to look at this is to look at vectors normal to the planes...two planes are parallel if and only if their normal vectors are parallel, so a sufficient and necessary condition for the intersection of two planes to be a line is that their normal vectors not be parallel.

Answer 3

TRUE!!! it says intersection, so it has to be a line

have yourself a good day.