In the book which I am reading/understanding, there is an integral(actually it is part of gauss' divergence theorem proof),
$ \int_{f_1(x,y)}^{f_2(x,y)}\frac{\partial F_3}{\partial z}\;dz $
Now, they have solved it this way,
$ =F_3\{x,y,f_2(x,y)\}-F_3\{x,y,f_1(x,y)\}\;\;-(i) $
Now, I am not able to understand how they did it. Till, now, I have learnt to directly put the upper end and lower limit. For example,
$ \quad \vert x\vert_1^2 = 2-1;\;\;-(ii) $
But, in the above case eq(i), they have somewhat not proceeded like eq(ii), Now can anyone please explain what exactly is happening?