Question

Cum efectuez (3+6+9+...+30)-(2+4+6+...+20)?

Answer 1

$(3+6+9+....+30)=3(1+2+3+....+10)=3 \frac{10.11}{2}= \frac{330}{2}=165$
$(2+4+6+....+20)=2(1+2+3+.....+10)=2 \frac{11.10}{2}= \frac{220}{2}=110$
165-110=55