\(\text{A= 1-2-3+4+5-6-7+8+9-...+1992+1993-1994}\)

\(A=\left(1-2-3+4\right)+...+\left(1989-1990-1992+1992\right)+1993-1994\)

\(A=0+0+...+0+1993-1994\)

\(A=-1\)

\(A=1-2-3+4+5-6-7+8+...+1992+1993-1994\)

\(A=\left(1-2-3+4\right)+...+\left(1989-1990-1991+1992\right)+1993-1994\)( 498 nhóm dư 2 )

\(A=0+0+...+0+1993-1994\)

\(A=1993-1994=-1\)

Vậy A = -1

Ta có :

A = 1-2-3+4+5-6-7+8+9-.....+1992+1993-1994

=> A = (1-2-3+4) + (5-6-7+8) + .....+ (1989-1990-1991+1992) + 1993 - 1994

= 0 + 0 + ...... + 0 + 1993 - 1994

= 0 + 1993 - 1994

= -1

Vậy A = -1

A=1-2-3 +4+5-6-7+8 +.....+1993

\(A=A_1+1993\)

\(A_1=\left(1-2-3+4\right)+\left(5-6-7+8\right)+....+\left(1989-1990-1991+1992\right)\)\(A_1=0+0+0...+0\)

A=1993

\(A=1-2-3+4+5-6-7+8+...+1989-1990-1991+1992+1993\)

\(A=\left(1-2-3+4\right)+\left(5-6-7+8\right)+...+\left(1989-1990-1991+1992\right)+1993\)

\(A=0+0+...+0+1993\)

\(A=1993\)

(1-2-3+4)+....+(1989-1990-1991+1992)+3985=3985

1+2+3+4+5+6+7+8+9+1+2+3+4+5+6+7+8+9+1+2+3+4+5+6+7+8+9+1028=(1+2+3+4+5+6+7+8+9)x3+1028=45x3+1028=135+1028=1163

= ( 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 ) x 3 +1028

\(=\) \(\frac{9.\left(9+1\right)}{2}\times3+1028\)

\(=45\times3+1028\)

\(=135+1028=1163\)

1163.

Chọn mk nha😊

b) 1-2-3+4+5-6-7+8+......+1989-1990-1991+1992+1993

=(1-2-3+4)+(5-6-7+8)+.....+(1989-1990-1991+1992)+1993

=0+0+...+0+1993=1993.