S1 + S2 = (1-2-3+4) +(5-6-7+8) + (9-10-11+12)+(13-14-15+16) +17 -18
= 0 + 0 + 0 + 0 + 17 -18
= -1
S1+S2=(1-2-3+4)+(5-6-7+8)+(9-10-11+12)+(13-14-15+16)+17-18
=0+0+0+0+17-18
=-1
trả lời :
-1
S1 + S2 = ( 1 - 2 - 3 + 4 ) + ( 5 - 6 -7 + 8 ) + ( 9 - 10 -11 + 12 ) + ( 13 - 14 - 15 + 16 ) + 17 - 18
= 0 + 0 + 0 + 0 + 17 - 18
= -1