Question

Determinati x€IN* astefel incat 21/x+1 este numar natura

Answer 1

21/(x+1)∈N<=>x+1 I 21

O fractie e numar natural daca si numai daca numitorul divide numaratorul.

x+1 | 21=>x+1∈D₂₁, D₂₁={1, 3, 7, 21}

x+1∈{1, 3, 7, 21} |-1

x∈{0, 2, 6, 20}

S={0, 2, 6, 20}