Question

Cum se rezolva exercitiul 12+24+36+.....+144 ?

Answer 1

$\displaystyle 12+24+36+...+144=12(1+2+3+...+12)= 12 \times \frac{12(12+1)}{2} = \\ \\ =12 \times \frac{12 \times 13}{2} =12 \times \frac{156}{2} =12 \times 78=936$