Thanks for your support.
I have one more question.
How can I get f, theta and phi values?
The following commands didn’t display the results for f(eta), theta(eta) and phi(eta).
Also, N[b] = 0.1 is probably not what you meant. N is a system function, and N[b] = 0.1 sets an NValue for the symbol b. If you don’t know what that means, then it is certainly not what you intended. I usually avoid starting my variable and function names with capital letters, and I especially avoid using single capitals, since several are used by the system. Like N. Sometimes when I want the variable to resemble traditional notation found in journals, I will compromise and use a double letter, like NN[b] = 0.1; similarly for, say, GG[r] = 5. But that’s up to you. Just know that some things such as N[t] = 0.1 have a special meaning and may break your code.