A:Every dog has its day.
B:A new broom sweeps clean.
C:A cat may look at a king.
D:A person is known by the company he keeps.
下列命题正确的有 ( )个①40°角为内角的两个等腰三角形必相似②若等腰三角形一腰上的高等于腰长的一半,则这个等腰三角形的底角为750③一组对边平行,另一组对边相等的四边形是平行四边形④一个等腰直角三角形的三边是a、b、c,(a>b=c),那么a2∶b2∶c2=2∶1∶1⑤若△ABC的三边a、b、c满足a2+b2+c2+338=10a+24b+26c,则此△为等腰直角三角形。