Question

Calculați va rog 1+3+5+...+19

Nu știu ajutatima

Answer 1

19 =2n-1
S = n*n

1+3+5+...+19=1+3+5+...+(2*10-1)
                    =10*10
                    =100

Answer 2

$\displaystyle 1+3+5+...+19= \\ \\ =1+2+3+4+5+...+19-(2+4+6+...+18)= \\ \\ = \frac{19(19+1)}{2} -2(1+2+3+...+9)= \frac{19 \times 20}{2} -2 \times \frac{9(9+1)}{2} = \\ \\ = \frac{380}{2} -2 \times \frac{9 \times 10}{2} =190-\not2 \times \frac{90}{\not2} =190-90=100$