课时训练·基础达标
1、C
2、A
3、D
4、105 cm和45 cm
5、5和20
6、1/2
7、解:∵ △ABC ∽ △A'B'C',设△A'B'C'另两边长分别为x,y,
∴ 5/15 = 4/x = 3/y ,
∴ 5x = 15 × 4,∴ x = 12.
∴ 5y = 3 × 15, ∴ y = 9.
∴ △A'B'C'的周长为l2 + 9 + 15 = 36.
8、解:(1)△ASR ∽ △ABC.
理由:∵ 四边形PQRS是正方形,∴ SR∥BC
∴ △ASR ∽ △ABC.
(2)由(1)知△ASR ∽ △ABC.
∴ AE/AD = SR/BC.设正方形PQRS的边长为x cm,
则AE = (40 - x)cm,
∴ (40-x)/40 = x/60,解得 x = 24 .
∴ 正方形PQRS的边长为24 cm.
9、解:(1)过点 F 作 FM∥AC,交BC于点M,如图D-27-3所示,
∵ F为AB的中点,
∴ M为BC的中点,
∴ FM = 1/2AC
由FM∥AC.得∠CED = ∠MFD,∠ECD = ∠FMD
∴ △FMD ∽ △FCD.∴ EC/FM = DC/DM = 2/3,
∴ EC = 2/3FM = 2/3 × 1/2AC = 1/3AC,
∴ AE/AC = (AC-EC)/AC = (AC-1/3AC)/AC = 2/3.
(2)∵ AB = a,∴ FB = 1/2AB = 1/2a.
又FB = EC,∴ EC = 1/2a.
∵ EC = 1/3AC,∴ AC = 3EC = 3/2a.