18．(a 2b)(a 2b)
1
b(a 8b) 2
a2 4b2 a2
1
·························································································· 6分 ab 4b2 ·
2
1
············································································································· 8分 ab ·
2
19．设 A x（度），则 B x 20， C 2x．
根据四边形内角和定理得，x (x 20) 2x 60 360． ········································ 4分 解得，x 70．
∴ A 70 ， B 90 ， C 140 ． ····································································· 8分
20．（1）B机器的产量为150件， ·················································································· 2分
A机器的产量约为210件． ·············································································· 4分
（2）C机器产量的百分比为40%． ················································································ 6分
设C机器的产量为x， 由
150x
，得x 240，即C机器的产量为240件． ···································· 8分
25%40%
21．（1）∵AE⊥BC，AF⊥CD，∴∠AEB=∠AFD=90°． ·············································· 2分 ∵四边形ABCD是平行四边形，∴∠ABE=∠ADF． ···················································· 4分 ∴△ABE∽△ADF ···································································································· 5分 （2）∵△ABE∽△ADF， ∴∠BAG=∠DAH．