1.4 Lösning 7c
Från Mathonline
\( {1\,-\,x\,y \over (x\,y)^2\,-\,x\,y} \; = \; {1\,-\,x\,y \over x\,y\,\cdot\,(x\,y\,-\,1)} \; = \; {(-1)\,\cdot\,(-1\,+\,x\,y) \over x\,y\,\cdot\,(x\,y\,-\,1)} \; = \; \)
\( = \; {(-1)\,\cdot\,(x\,y\,-\,1) \over x\,y\,\cdot\,(x\,y\,-\,1)} \; = \; {(-1) \over x\,y} \; = \; - {1 \over x\,y} \)
