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2 2/1:×=7.5

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x:7.5=1/2 x=7.5×1/2 x=3.75

28一x=15 x=28一15 x=13

7又1/2:1.2=2.5/x等式计算 解: 7又1/2:1.2=2.5/x 7.5x=1.2*2.5 7.5x=3 x=0.4

=7.5×2.1+7.8×7.9 =7.5×2.1+(7.5+0.3)×7.9 =7.5×2.1+7.5×7.9+0.3×7.9 =7.5×(2.1+7.9)+2.37 =7.5×10+2.37 =75+2.37 =77.37

原式= (1+2+...+9) -(1/2+2/2+3/2+...+8/2) +(1/3+2/3+...+7/3) -(1/4+2/4+...+6/4) +(1/5+...+5/5) -(1/6+...+4/6) +(1/7+2/7+3/7) -(1/8+2/8) +1/9 =45-36/2+28/3-21/4+15/5-10/6+6/7-3/8+1/9 =33+5/504

解:原式=7.5×2.1+7.8x7.9 =7.5×2.1+(7.5+0.3)x7.9 =7.5×2.1+7.5x7.9+0.3x7.9 =7.5×(2.1+7.9)+0.3x(8-0.1) =75+2.4-0.03 =77.37

7.5×2.1+2.1×2.5+12.5×0.4 =2.1×(7.5+2.5)+50 =21+50 =71

x:2分之1=3分之2:4分之3 3/4x=2/3×1/2 3/4x=1/3 x=4/9 10-x分之7=2.5分之6 6(10-x)=7×2.5 60-6x=17.5 6x=42.5 x=85/12 25%:7.5=x:2又2分之1 7.5x=2.5×25% 7.5x=0.625 x=1/12

7.5X2.1+75X0.79 =7.5*2.1+7.5*7.9 =7.5*(2.1+7.9) =7.5*10 =75

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