Vom da factor comun pe 3¹⁸
[tex]\displaystyle\bf\\a=\big(3^{21}+3^{20}+3^{19}\big):39\\a=\big(3^{18+3}+3^{18+2}+3^{18+1}\big):39\\a=\big(3^{18}\times3^3+3^{18}\times3^2+3^{18}\times3^1\big):39\\a=3^{18}\big(3^3+3^2+3^1\big):39\\a=3^{18}\times(27+9+3):39\\\\a=\frac{3^{18}\times(27+9+3)}{39}\\\\a=\frac{3^{18}\times39}{39}\\\\a=3^{18}=\big(3^9\big)^2=pp[/tex]
Am simplificat fractia nu 39 si a ramas 3¹⁸
care e patrat perfect deoarece exponentul 18 este numar par.
