## Calculus 8th Edition

True $f_{y}(a,b)=\lim\limits_{y \to b}\frac{f(a,y)-f(a,b)}{y-b}$ Suppose $h=y-b$ Then $f_{y}(a,b)=\lim\limits_{h \to 0}\frac{f(a,b+h)-f(a,b)}{h}$ This is accordance to the definition of Limits and continuity.