I'm working with the complex equation in figure (Prova1). This equation is function of different variables. If I evaluate the equation in all the variables except A and wel4, I obtain an equation in function of A and wel4 (Prova2). If now evaluate this second expression in A = 1 and wel4 = -1 setting the precision to 64 digits, I obtain a result (Prova3). If I evaluate the same expression without setting the precision, the result is different (Prova4).

If now I try to replace in the equation Prova1 all the variables except A, I obtain an equation in function only of A (Prova5). If I evaluate this equation in A = 1, the final result is different from those obtained previously.

Can someone help me to understand why it happens? Thanks