In △BAD and △BAC,

AB=AB

∠BAD=∠BAC=90

AD=AC

∴△BAD≌△BAC(SAS)

∴BD=BC

Page 15: 3. In △ABE and △ACD,

AB=AC

∠A=∠A

AE=AD

∴△ABE≌△ACD

4. just measure the distance of A'B'

Reason: ∫ Point O is the midpoint between AA' and BB'.

∴ao=a′o

BO = B′O

In △AOB and △ a ′ ob ′.

AO = A′O

∠AOB =∠A′OB′

BO = B′O

∴△aob≌△a′ob′

∴a′b′=ab