l) x/(x+1)+1/(x+1)=
=(x+1)/(x+1)=1
m) (y+1)/(y+3)+3/(y+3)+(y+2)/(y+3)=
=(y+1+3+y+2)/(y+3)=
=(2y+6)/(y+3)=
=2(y+3)/(y+3)=2
n) 2/(n+1)+4/(n+1)+6/(n+1)+....+2n/(n+1)=
=(2+4+6+....+2n)/(n+1)=
=2(1+2+3+...+n)/(n+1)=
=2×[n(n+1)/2]/(n+1)=
=n(n+1)/(n+1)=n
