课堂检测
6、解:AB/AD = AC/AE,理由:
∵ ∠DAB = ∠CAE,
∴ ∠DAE = ∠BAC.
又∵ AB/AD = AC/AE
∴ △ABC ∽ △ADE.
7、解:如图D-27-2所示,A,B为相距 50 m 的两根电线杆,
C,D是两棵树,它们之间还有 3 棵树,
故CD = 4 × 5 = 20(m),OE = 25 m.
∵ CD ∥ AB
∴ △OCD ∽ △OAB.
△OCE ∽ △OAF,
即25/(25+EF) = 20/50,解得EF = 37.5.
答:河宽为37.5 m.
8、证明:∵ A′B′∥ AB
∴ △OA′B′∽ △OAB,
∴ △ABC ∽ △A′B′C′