wow my book is terrible and doesn't mention these types of problems...just assigns them.

find the constant c for to make the following pdf function.

a. f(x) = cx where x = 1, 2, ... , 25.

b. f(x) = c(x+1)^3 where x = 0,1,2

c. f(x) = c(1/3)^x where x= 1, 2, 3, ...

i know what its asking for but i have absolutely no idea where to begin solving for c.