56.5-41.2+43.5-58.8 =(56.5+43.5)-(41.2+58.8)=0
25×125×5×2×4×8=(5^2)X(5^3)x5x2x(2^2)x(2^3)=(5^6)x(2^6)=(5x2)^6=10^6
25×1.6=25x4x0.4=100x0.4=40
125×72=125x8x9=1000x9=9000
99×45=(100-1)x45=100x45-1x45=4500-45=4455
101×45=(100+1)x45
9.9×45+4.5=99x4.5+4.5=100x4.5
101×45-45=(100-1)x45
9999+999+99+9=(10000-1)+(1000-1)+(100-1)+(10-1)=11110-4=11106
9999×2222+3333×3334=3x3333x2222+3333x3334=3333x(3x2222+3334)=3333x10000
10000-1-2-3-4-...-80=10000-(1+2+3+4+...+80)=10000-[(80+1)X80]/2=10000-3240=6760
1+2+3+4+...+78+79+80=[(80+1)X80]/2=3240
都是一个一个打出来的,好辛苦,多给点分吧~