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2.3x12,怎么用竖式计算

2.3x12=27.6 竖式如下:

0.376x27.3+2.73x8.54+7.27x12.3 =0.376x27.3+27.3x0.854+7.27x12.3 =(0.376+0.854)x27.3+7.27x12.3 =1.23x27.3+72.7x1.23 =1.23x(27.3+72.7) =1.23x100 =123

3.2 X 12.5 = 320 X 0.125 = 40 X 8 X 1/8 = 40

设x1,x2,…xn中有r个-1,s个1,t个2,则?r+s+2t=19r+s+4t=99,得3t+s=59,0≤t≤19,∴x13+x23+…+xn3=-r+s+8t=6t+19,∴19≤x13+x23+…+xn3≤6×19+19=133,在t=0,s=59,r=40时,x13+x23+…+xn3,取得最小值19,在t=19,s=2,r=21时,x13+x23+…+xn3=9...

 7/9x(-8/9)+2/9x(-8/9)=(-8/9)×(7/9+2/9)=-8/92/3x(-5/8)-(-1/3)x(-5/8)=(-5/8)× (2/3+1/3)=-5/8-9÷[3+(1/2-2/3)x12+3 2]=-9÷[3+(-4)+9] =9/8 

3.2x12.5x7x2.5 =4x0.8x12.5x2.5x7 =4x2.5x0.8x12.5x7 =10x10x7 =700

(2/3+5/12)x12 =2/3×12+5/12×12 =8+5 =13 5/9x(8/13x9/10) =(5/9×9/10)×8/13 =1/2×8/13 =4/13 2/9-7/16x2/9 =(1-7/16)×2/9 =9/16×2/9 =1/8 11/8x4/7-0.375x4/7 =11/8×4/7-3/8×4/7 =(11/8-3/8)×4/7 =1×4/7 =4/7

解: 设第n项为an an=n(n+1)(n+2)=n^3+3n^2+2n 1×2×3+2×3×4+...+10×11×12 =(1^3+2^3+...+10^3)+3(1^2+2^2+...+10^2)+2(1+2+...+10) =[10(10+1)/2]^2+3×10×(10+1)(20+1)/6+2×10×11/2 =3025+1155+110 =4290 用到的公式: 1^3+2^3+...+n^3=[n(n+1)/...

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