s1 question

when you're working out sxx or syy, you have sumx^2 - (sumx)^2/n.
for the second half you do work out the square of sumx and then divide that by n or do you work out sumx/10 and then square it. Because i thought it was affectivly working out the square of the mean, but if you do it the first way thats not the square of the mean..and the first way seems to be the right one.

Also i was jsut wondering if the exam (edexcel) is marked positively...as in if we dont know what method to use for a question can we write two and will the right one get the credit?
thanks

Reply 1

Juncture

its the first way, i don't think it's supposed to be working out the square of the mean so don't think about it that way, if you have to remember it (I'm on OCR so it's in our formula booklet) i'd say the easiest way to remember is to learn Sxy cause to me Sxx and Syy make more sense after that

Reply 2

**12345**

thank you!! no we dont have to remember it i was just getting a bit confused....hehe

also do you by any chance know what it means when theres a question about discrete random variables and it starts talking about E1 and E2 and "observations"

for example i have a question where one part goes like:
two independant observations are made X1 and X2. show that P(X1+X2=5)=0 and find out the complete probability function for X1+X2 and find P(1.3<X1+X2<3.2)

Ive never seen X1 and X2 before in my life!!