Question

Rezolvati folosind regula factorului comun:(am invatat si despre suma lui Gauss
11+22+33+...+99=

Answer 1

$\displaystyle 11+22+33+...+99=11(1+2+3+...+9)=11 \times \frac{9(9+1)}{2} = \\ \\ =11 \times \frac{9 \times 10}{2} =11 \times \frac{90}{2} =11 \times 45=495$