Question

S=2+4+6+8+.....+238+239+240

Answer 1

S=2+4+6+8+.....+238+240
S=2* (1+2+3+4+...+119+120)
S=2*$\frac{120*121}{2}$   (doi cu doi se simplifica)
S=120*121
S=14520

Answer 2

$\displaystyle 2+4+6+8+...+238+240=2(1+2+3+...+119+120)= \\ \\ =2 \times \frac{120(120+1)}{2} =2 \times \frac{120 \times 121}{2} =\not 2 \times \frac{14520}{\not 2} =14520$