Răspuns:
1395
Explicație pas cu pas:
Salut! (●'◡'●)
[tex]3+6+9+...+90=\\[/tex]
Dam factor comun pe 3. Obtinem:
[tex]3(1+2+3+...+30)=\\[/tex]
Folosim suma Gauss:
[tex]3\cdot \frac{30\cdot31}{2}\\\\3\cdot 15\cdot31\\\\45\cdot31\\\\\underline{1395}[/tex]
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