A^2=[1/2(B+E)]^2=1/4(B^2+2B+E)=A=1/2(B+E)化简得B^2=E
∵A²=1/4*（B²+2B+E）=B²/4+B/2+E/4=B/2+E/2∴B²/4=E/4即B²=E
