Help with cdf

#1
I'm having trouble with grasping cdf's. If I have a cumulative distribution function
F(x) =
.0 ----- x<-2
.2 ----- -2 ≤ x< 0
.5 ----- 0 ≤ x < 1
.7 ----- 1 ≤ x < 3
.9 ----- 3 ≤ x < 4
1.0 ----- x ≥ 4
Im trying to evaluate P(-1 < X < 3) and P(X=4). For P(-1 < X < 3) im doing P(3≤ X<4)-P(-2≤ X<0) which gives me .7. For P(X=4) I subtract P(3<=X<4) from 1 giving me 1-.9=.10.
Im not sure I am doing this right though.
 
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#4
Yes, positive 1 is included in P(1<=X<3)

Just so we're on e same page, this is a cumulative distribution function of a discrete probability distribution.

The graph of the cdf looks like this



From the graph you should be able to calculate the pdf
 
#5
Yes, positive 1 is included in P(1<=X<3)

Just so we're on e same page, this is a cumulative distribution function of a discrete probability distribution.

The graph of the cdf looks like this



From the graph you should be able to calculate the pdf
Right so the pmf values would be the difference between the two points (vertically) for that number. Unless it is undefined.