分析:(1)根據(jù)等式的性質(zhì),兩邊同減去2.7即可;
(2)原式變?yōu)?.8+2.8x=21,根據(jù)等式的性質(zhì),兩邊同減去9.8,再同除以2.8即可;
(3)根據(jù)等式的性質(zhì),兩邊同加上6.3,再同除以4.8即可;
(4)原式變?yōu)?.6x=55.2x-993.6,根據(jù)等式的性質(zhì),兩邊同減去4.6x,得50.6x-993.6=0,兩邊同加上993.6,再同除以50.6即可;
(5)根據(jù)等式的性質(zhì),兩邊同乘,4.5,得x-18=10.8,兩邊同加上18即可;
(6)原式變?yōu)?.2+0.8x=20,兩邊同減去5.2,再同除以0.8即可.
解答:解:(1)x+2.7=12,
x+2.7-2.7=12-2.7,
x=9.3;
(2)2.8×(3.5+x)=21,
9.8+2.8x=21,
9.8+2.8x-9.8=21-9.8,
2.8x=11.2,
2.8x÷2.8=11.2÷2.8,
x=4;
(3)4.8x-6.3=68.1,
4.8x-6.3+6.3=68.1+6.3,
4.8x=74.4,
4.8x÷4.8=74.4÷4.8,
x=15.5;
(4)8.5x-3.9x=55.2(x-18),
4.6x=55.2x-993.6,
4.6x-4.6x=55.2x-993.6-4.6x,
50.6x-993.6=0,
50.6x-993.6+993.6=993.6,
50.6x=993.6,
50.6x÷50.6=993.6÷50.6,
x=19.
;
(5)(x-18)÷4.5=2.4,
(x-18)÷4.5×4.5=2.4×4.5,
x-18=10.8,
x-18+18=10.8+18,
x=28.8;
(6)0.65×8+0.8x=20,
5.2+0.8x=20,
5.2+0.8x-5.2=20-5.2,
0.8x=14.8,
0.8x÷0.8=14.8÷0.8,
x=8.5.
點(diǎn)評:此題考查了運(yùn)用等式的性質(zhì)解方程,即等式兩邊同加上或同減去、同乘上或同除以一個數(shù)(0除外),兩邊仍相等,同時注意“=”上下要對齊.