# P() is probability of standard normal distribution; 

P(x_<b_):=Phi(left(solve(x-b,x)));
P(x_<=b_):=Phi(left(solve(x-b,x)));
P(x_>a_):= 1-Phi(left(solve(x-a,x)));
P(x_>=a_):= 1-Phi(left(solve(x-a,x)));

P(a_<x_<b_):=Phi(solve(x-b,x))-Phi(solve(x-a,x));
P(a_<=x_<=b_):=Phi(solve(x-b,x))-Phi(solve(x-a,x));