Bai 1:
a)Xet tam giac goc ADF = goc ABE(=90)
AD=AB(gt)
DF=BE(gt)
=>tam giac ADF = tam giac ABE(ch-cgv)
=>AE=AE.    (1)
=>goc DAF = goc BAE
Ma goc DAF+ goc FAB=90
=>goc BAE + goc FAB =90       (2)
Tu (1) va (2)=>tam giac AEF vuong can