Question

1+2+3......+10
rapid repede

Answer 1

1+2+3+......+10 = (1+10)·10:2 = 11·5 = 55

Answer 2

$1 + 2 + 3 + ... + n = \frac{n(n + 1)}{2} \\ \\ 1 + 2 + 3 + ... + 10 = \frac{10(10 + 1)}{2} = \frac{10 \times 11}{2} = 5 \times 11 = 55$