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# S2 Continuous random variables watch

1. When you integrate a pdf to get a cdf, how do you know when you need a constant and how do you work the constant out? I'm confused and this is ruining my revision timetable so any help would be appreciated!
2. I'm actually unsure about this as well. A way to check that a constant is needed, however, would be to sub your limits back into their corresponding expression, and adding it all up to see if it equals 1. If so, then you're ok. If not, then obviously there's a constant missing. I need help on this too...
3. If you find K to start of with, there is no constant.

Basically, all the different parts of the CDF HAVE to meet each other and the CDF must never go above one.

Take a look at this example i did for someone....

PDF

I know my handwriting is crap so just tell me if it doesn't make sense.
Attached Images
4. pdf.pdf (43.6 KB, 75 views)
5. (Original post by K2theJ)
When you integrate a pdf to get a cdf, how do you know when you need a constant and how do you work the constant out? I'm confused and this is ruining my revision timetable so any help would be appreciated!
If you have a pdf f(x) then you don't really need to use a constant for the cdf F(x) if you define it as

6. (Original post by K2theJ)
When you integrate a pdf to get a cdf, how do you know when you need a constant and how do you work the constant out? I'm confused and this is ruining my revision timetable so any help would be appreciated!

when u integrate sumthing ur finding its area... and u r always given limits between which the pdf exist - say 1<x<8

when u integrate ur pdf & get the cdf with the constant C ,
the---

cdf +C = 0 when u substitute ur 'x' as 1 ( the lower limit ) cuz ur area here is nill

cdf +C = 1 when u substitute ur 'x' as 8 ( the upper limit ) cuz ur area here by the end of ur graph wld b 1 ( hope im making sense !!)

that how i remember it & it has worked till now.. u can use either substitution .. hope it helps !
7. thanks so much guys! that really helps!

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