Why do some equations have one solution, others two or even more solutions and some no solutions? Why do we sometimes need to ""switch"" the direction of an ine

Are sequences functions? Why can’t the popular “vertical line test” be applied in some cases to determine if a relation is a function? How does the idea o

"A series for teaching mathematics."--P. [1] of cover.

Why are there so many formulas for area and volume, and why do some of them look alike? Why does one quadrilateral have no special name while another has severa