Question

(2+4+6+.......+20)-(1+3+5+.....+19)

Answer 1

(2+4+6+.......+20) - (1+3+5+.....+19) =

= 2-1 + 4-3 + .. + 20-19

= 1 + 1 + .. + 1   | x 10

= 10

Answer 2

$\it (2+4+6+...+20) - (1+3+5+...+19) =2(1+2+3+\ ...\ +10)-10^2=\\ \\ =\not2\cdot\dfrac{10\cdot11}{\not2}-100 = 110-100=10$