(u/v)'=(u'v-uv')/v²
u=(х+5); v=(х-1); (x-1)'=1; (x+5)'=1;
((х+5)/(х-1))'=((x+5)'*(x-1)-(x-1)'*(x+5))/(x-1)²=(x-1-x-5)/(x-1)²=-6/(x-1)²
