L'hopital's rule problem

i don't how my text book gets - 1

explain and showing work would

Apublic library wants to place 4 magazines and 9 books on each display shelf. the expression 4s+9s represents the total number of items that will be displayed on s shelves. simplify the expression

1) prove that 731^3−631^3 is divisible by 100 2) prove that 99^3−74^3 is divisible by 25

Consider the sequence below. 3 , 1 , 1/3 , 1/9 , select the explicit function which defines the sequence.