【急】 1/8减2/9加7/8减7/9等于多少

1/8 -2/9 +7/8 -7/9
= (1/8+7/8) -(2/9+7/9)
= 1 -1
= 0
20+1/9 -1/8 -5/8 -1/4
= 20 +1/9 -(1/8+5/8+1/4)
= 20 +1/9 -(6/8+1/4)
= 20 +1/9 -(3/4+1/4)
= 20 +1/9 -1
= 19+1/9
= 19又 1/9
1/2 -x +3/4 =3/8
两边同时乘以8,得 4 -8x+6 =3
可得 x = 7/8
2x+1/12 =11/12
移项,得 2x = 11/12 -1/12
即 2x = 5/6
得 x = 5/12
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