Question

b=2+4+6+.....+78+80 va rog ajutati-ma..

Answer 1

$\boxed{\boxed{1+2+3+...+n = \dfrac{n\times(n+1)}{2}}} \rightarrow Suma lui Gauss \\ \\ \\ B = 2+4+6+...+78+80 = 2\times(1+2+3+...+39+40) = \\ \\ =2\times\dfrac{40\times(40+1)}{2} = 2\times \dfrac{40\times 41}{2} = 40\times 41 = 1640$

Answer 2

B=2+4+6+.....+78+80  = 2 x (40 x 40+1) : 2 =  40 x 41 = 1640