(1)-1+2-3+4-5+6-7;(2)-50-28+(-24)-(-22);(3)-19.8-(-20.3)-(+20.2)-10.8;(4)0.25-+(-1)-(+3).

更新时间：2022-03-30 16:16:58 73次访问

-1+2-3+4-5+6-7

=-1+(2-3)+(4-5)+(6-7)

=-1×4

=-4

//

-50-28+(-24)-(-22)

=-78-24+22

=-80

//

-19.8-(-20.3)-(+20.2)-10.8

=-19.8+20.3-20.2-10.8

=0.5-31

=-30.5

//

0.25-+(-1)-(+3)

=0.25+1-3

=-1.75

//

2x+5y=-21①  x+3y=8②

②×2-①  y=37③

②-③×3  x=-103

(x，y)=(-103，37)

//

8s+9t=23①  17s-6t=74②

16s+18t=46③  51s-18t=222④

③+④  67s=268  s=4⑤

①-⑤×8  9t=-9  t=-1

(s，t)=(4，-1)