=2×(-27)-4×(-3)+15
=-54-(-12)+15
=-54+12+15
=-54+27
=-(54-27)
=-27;
(2)(
1 |
2 |
5 |
9 |
7 |
12 |
=(-36)×
1 |
2 |
5 |
9 |
7 |
12 |
=-18+20+(-21)
=[-18+(-21)]+20
=-39+20
=-(39-20)
=-19;
(3)∵1+2+3+…+31+32+33=17×33,
∴1-3+2-6+3-9+4-12+…+31-93+32-96+33-99
=-2(1+2+3+…+31+32+33)
=-2×17×33
=-1122.