Question

S2=6+9+12+15+... +60
repede cu explicații dau coroana ​

Answer 1

 

$\displaystyle\bf\\S=6+9+12+15+...+60\\\\Calculam~numarul~de~termeni:\\\\n=\frac{60-6}{3}+1=\frac{54}{3}+1=18+1=19~termeni\\\\Calculam~suma~cu~formula~lui~Gauss.\\\\S=6+9+12+15+...+60=\frac{n(60+6)}{2}=\frac{19\times66}{2}=19\times33=\boxed{\bf627}$