# Homework Help: Engineering Joint probability problem

1. Jul 10, 2010

### Spartan452

1. The problem statement, all variables and given/known data

axial load in an interior column in a two-story building is a random variable Y described as the sum of the floor loads X1 and X2 the floor load each is a normally distributed random variable with a mean value of 90kn.m and a coefficient of variation of .20. The random variables X1 and X2 are independent.

A)-Compute the mean and std deviation of Y
B)-If the resistance of the column R is a normal random variate with a mean of 270KN and a coefficient of variation of .20, compute the probability of failure of the column assuming that Y and R are statistically independent

2. Relevant equations

Im just kinda confused on where to get started, I believe the function is continuous but I don't know what the bounds are nor do I know what the function actually looks like.
It says the function is normally distributed so I think it would be the standard normal density function phi(x-mean/std dev) and since there are two and they are independent it should be squared. I do not know what the target value is either. But i might be wrong and my approach might be totally off, my book doesn't really provide any examples on how to deal with a problem like this...

I think you have to solve b the same way you would solve for a, but I just don't know how to deal with a.

3. The attempt at a solution
This is my attempt attached here, I think it might all be wrong...

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