Question

cum se procedeaja la suma lui gaos daca am asa??
a=3+6+9+...........+75-75=

Answer 1

3*1+3*2+3*3+...+3*25-75=3(1+2+3+...+25)-75=3(1+25)25/2- 75=3*13*25=75=75*13-75=75(13-1)=75*12=900

Answer 2

$a=3+6+9+...+75-75=3(1+2+...+25)-75=\frac{3*25*26}{2}-75=900$