小学数学真题12 15 3

所以1/12+2/12+3/12+. 239/12。

= (1 +2+3.+239)/12 -(1+2+3.+19)

=((1+239)X239)/(2x 12)-((19+1)x 19)/2

=2390 - 190

=2200

2.3321 /20 ÷ 41 =(3321 /41)/20= 81/20

3.(-1/3+7/12-9/20+11/30-13/42+15/56)×21

=-1/3+(3+4)/(3 x4)-(4+5)/(4x 5)+(5+6)/(5x 6)-(6+7)/(6x 7)+(7+8)/(7x 8))×21

=(-1/3+(1/3+1/4)-(1/4+1/5)+(1/5+1/6)-(1/6+1/7)+(1/7+1/8))×21

=21/8

=2.625