n2+3⋮n+1n2+3⋮n+1
⇒n2+n+3−n⋮n+1⇒n2+n+3−n⋮n+1
⇒n(n+1)−n+3⋮n+1⇒n(n+1)−n+3⋮n+1
Vì n(n+1)⋮n+1n(n+1)⋮n+1
nên −n+3⋮n+1−n+3⋮n+1
⇒−n−1+4⋮n+1⇒−n−1+4⋮n+1
⇒−(n+1)+4⋮n+1⇒−(n+1)+4⋮n+1
Vì −(n+1)⋮n+1−(n+1)⋮n+1
nên 4⋮n+14⋮n+1
⇒n+1∈Ư(4)={±1;±2;±4}⇒n+1∈Ư(4)={±1;±2;±4}
⇒n∈{0;−2;1;−3;3;−5}⇒n∈{0;−2;1;−3;3;−5}