(1)-1+2-3+4-5+6-7;(2)-50-28+(-24)-(-22);(3)-19.8-(

-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=8②②×2-① y=37③②-③×3 x=-103(x，y)=(-103，37)//8s+9t=23① 17s-6t=74②16s+18t=46③ 51s-18t=222④③+④ 67s=268 s=4⑤①-⑤×8 9t=-9 t=-1(s，t)=(4，-1)