Question

Efectuati ( 3+6+9+....+30)-(2+4+6+.....+20)

Answer 1

(3+6+9+.......+30)=3x(1+2+3+........+10)=10x(10+1):2x3=10x11:2x3=110:2x3=  55x3=165
(2+4+6+.........+20)=2x(1+2+3+.........+10)=10x(10+1):2x2=10x11:2x2=110:2x2=  55x2=110
165-110=55
sau mai simplu
(3+6+9+.......+30)(2+4+6+.........+20)= 3x(1+2+3+........+10)-2x(1+2+3+.....+10)=1+2+3+...+10=
10x(10+1):2=10x11:2=110:2=55

Answer 2

     
$(3+6+9+ \hdots +30)-(2+4+6+ \hdots +20)= \\ =3(1+2+3+ \hdots +10)-2(1+2+3+ \hdots +10)= \\ \\ =(1+2+3+ \hdots +10)(3-2)= \\ \\ =(1+2+3+ \hdots +10)= \frac{10(10+1)}{2} = \frac{10\;\cdot \;11 }{2} = \frac{110 }{2}=\boxed{55}$