1+3-5-7+9+11-13-15+...-397-399

= 1 + (3-5-7+9) + (11-13-15+17)+...+ (397-399-401+403)+401-403

= 1+0+0+....+0+401-403

=1+401-403

=-1

1 + 3 - 5 - 7 + 9+11-13-15+...-397-399

có (399-1):2+1=200 chữ số 

=(1 + 3 - 5 - 7) + (9 + 11 - 13 - 15)+ ... + (393 + 395 - 397 - 399)

=(-8) + (-8) + ... + (-8)

có 200 : 4 = 50 số -8

=(-8) x 50

=-400

1+3-5-7+9+11-13-15+...-397-399=-400

giải thích rõ ràng nha m.n

=(1+3-5-7)+(9+11-13-15)+........+(393+395-397-399) =-8+(-8)+.....+(-8) có 50 so -8 =-8 .50=-400

Ta có : 1+3-5-7+9+11-13-15+..............+395-397-399

= (1+3-5-7)+(9+11-13-15)+.........+(393+395-397-399) ( 50 nhóm)

= (-8)+(-8)+................+(-8) ( 50 số)

= (-8). 50

= -400

SSH: (399-1):2+1= 200
Neu chia moi nhom 4 so thi so cap so la:
200:4 = 50
Ta co:
B=1+3-5-7+9+11-13-15+...+393+395-397-399
B= (1+3-5-7)+(9+11-13-15)+...+(393+395-397-399)
B= -8 + -8 +...+ -8
B= -8 . 50
B= -400

SSH: (399-1):2+1= 200
Neu chia moi nhom 4 so thi so cap so la:
200:4 = 50
Ta co:
A=1+3-5-7+9+11-13-15+...+393+395-397-399
A= (1+3-5-7)+(9+11-13-15)+...+(393+395-397-399)
A= -8 + -8 +...+ -8
A= -8 . 50

A= -400

Đặt biểu thức là A

Ta có

\(A=\left(1+3-5-7\right)+\left(9+11-13-15\right)+.....+\left(393+395-397-399\right)\)

\(\Rightarrow A=\left[\left(1-5\right)+\left(3-7\right)\right]+......+\left[\left(393-397\right)+\left(395-399\right)\right]\)

\(\Rightarrow A=\left[\left(-4\right)+\left(-4\right)\right]+.....+\left[\left(-4\right)+\left(-4\right)\right]\)

\(\Rightarrow A=\left(-8\right)+.....+\left(-8\right)\)

\(\Rightarrow A=\left(-8\right).50\)

\(\Rightarrow A=-400\)