Question

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

tính tổng

 

Answer 1

1+(-2)+3+(-4)+.......+19+(-20) \(=\left(1+19\right)+\left(-2+-18\right)+\left(3+17\right)+.....+\left(11+9\right)+\left(-10\right)+\left(-20\right)\)

\(=20-20+20-20+20-20+20-20+20-20-10-20\)

\(=-30\)

Answer 2

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

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

\(=-1-1-1-1-1-1-1-1-1-1\)

\(=-10\)

 

Answer 3

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

 =(1+19)+(−2+−18)+(3+17)+.....+(11+9)+(−10)+(−20)

=(1+19)+(−2+−18)+(3+17)+.....+(11+9)+(−10)+(−20)

=20−20+20−20+20−20+20−20+20−10−20

=−10

vừa ghi nhầm 1 số 20 nha !

Answer 4

Ta có: \(1+\left(-2\right)+3+\left(-4\right)+...+19+\left(-20\right)\)

\(=\left(1+19\right)-\left(2+18\right)+\left(3+17\right)-\left(4+16\right)+...-20\)

\(=20-20+20-20+...-20\)

=-20