Ответ:
а)х2-11х+28=х2-10х-х+25+3=(х-5)^2+(-х+3)
б)6х2+5х+1=5х2+х2+2х+3х+1=(х2+1)^2+5х2+3х
(х2-9)/(х2-2х-15)=((х-3)(х+3))/(х2+3х-5х-15)=((х-3)(х+3)/(х(х+3)-5(х+3)=((х-3)(х+3)/((х+3)(х-5))=(х-3)/(х-5)
(х/(х2-16)-(х+4)/(х2-3х-4))×(х2+5х+4)/(7х+16)=(х/((х-4)(х+4))-(х+4)/(х2+х-4х-4))×(х2+4х+х+4)/(7х+16)=(х/((х-4)(х+4))-(х+4)/(х(х+1)-4(х+1))×(х(х+4)+х+4)/(7х+16)=(х/((х-4)(х+4))-(х+4)/((х+1)(х-4))×((х+4)(х+1))/(7х+16)=(х(х+1)-(х+4)^2/((х+4)(х+1)(х+4))×((х+4)(х+1))/(7х+16)=(х2+х-(х+4)^2)/(х-4)×1/(7х+16)=(х2+х-(х+4)^2/((х-4)(7х+16))=(х2+х-(х2+8х+16))/((х-4)(7х+16))=(х2+х-х2-8х-16)/((х-4)(7х+16))=(-7х-16)/((х-4)(7х+16))=(-(7х+16))/((х-4)(7х+16))=(-1)/(х-4)