解：①如图，∵∠ABD=70°，∠BDA=90°，∴∠A=20°，∵AB=AC，∴∠C=∠ABC=（180°-20°）÷2=80°．②如图，∵∠ABD=70°，∠BDA=90°，∴∠BAD=20°，∵AB=AC，∴∠C=∠ABC=20°÷2=10°．故答案为：80°或10°．
