1. The problem statement, all variables and given/known data Dear all I am calibrating a temperature measurement model and I am stuck with an equation. The variable z is given; x and y represent two regression terms with common regressors - which I will solve for a specific regressor in a second step. 2. Relevant equations ln(e^x) = ln(z) + ln((e^x) + (e^y)) I need all x and y non-exponential, so that I can solve the equation for x and y. Now I am confused with the properties of ln((e^x) + (e^y)), this is a bracket of two exponential summands with two different exponents. 3. The attempt at a solution As far as I know, the term ln(a+b) can not be rewritten that easily. But I was wondering if this changes if both summands are exponential, as in my case. How can I solve the equation for x and y? Thank you.