Numerele √2,√5,√7 sunt in progresie aritmetica daca
√5 = (√2+√7)/2
2√5=√2+√7 si ridicam la patrat
(2√5)²=(√2+√7)²
20=2+2√14+7
20-2-7=2√14
11=2√14 si ridicam la patrat
11²=(2√14)²
121=4·14
121=56 (Fals)
Deducem ca √5 ≠(√2+√7)/2, adica numerele √2,√5,√7 nu pot fi termenii unei progresii aritmetice.