[R] Derivative of a function

[Lisa]

Dear all,

I just want to get the derivative of a function that looks like:

y = exp(x1*b) / (exp(x1*b) + exp(x2*b))

where y is a scalar, x1, x2, and b are vectors. I am going to take the
derivative of b with respect to y, but I cannot derive an expression in
which b is function of y. I know there is another way to get the similar
result, i.e., first take the derivative of y with respect to each element of
b, and then take its reciprocal. But it is not what I want. Could someone
please tell me how to solve this problem? Thank you in advance. 

Lisa


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