Question

1.△ABC中,AB=AC,∠BAC=120°,D为BC上一点,DA⊥AB,AD=24,则BC=?

Answer 1

1.BC=72

∠DAC=∠BAC-∠DAB=120-90=30°

∠CBA=∠BCA=(180°-120°)/2=30°=∠ACD

DA=DC=24

RT△BAD中 ∠ABD=30°

BD=2AD=48

BC=BD+DC=72

2.35°

证△ABP全等于△CBP

BP=BP BA=BC AP=PC

3.A

我用正弦定理

sinC/AB=sinB/AC

sinC/sinB=AB/AC

sin2B/sinB=AB/AC

sin2B=2sinB*cosB

AB/AC=2cosB

0°