1.因?yàn)锳/B=B/C=C/D=D/A,移項(xiàng),得:A^2=BD=B/C=C/D,再移項(xiàng),
得:A^2=BD=C^2=BD.
同理,D^2=AC=B^2=AC.
所以,A=±C,B=±D.
帶入方程,
(1)若,A=-C,B=-D,則=0;
(2)若A=C,B=D,則=(A-B)/B;若A=C,B=-D,則方程無(wú)意義.
(3)若A=-C,B=D,則=0
綜合以上:原式=0
2.（xy-1)^2-2x^2y-2xy^2+x^2+y^2+6xy-2x-2y
=x^2y^2+1-2xy-2x^2y-2xy^2+x^2+y^2+4xy+2(xy-x-y)
=(xy-x-y)^2+2(xy-x-y)+1
=(xy-x-y+1)^2
=(4+1)^2
=25
3.(a-c)/b=b/a,所以b^2=a^2-ac
c/(a+b)=b/a,所以ac=ab+b^2
兩式替換ac：
b^2=a^2-ab-b^2
a^2-ab-2b^2=0
(a-2b)(a+b)=0
a=2b或a=-b
把a(bǔ)=2b代入(a-c)/b=b/a
(2b-c)/b=b/2b
2c=3b
把a(bǔ)=-b代入(a-c)/b=b/a
(-b-c)/b=b/(-b)
c=0
選( A )
LS的第二問(wèn)答得好像有點(diǎn)問(wèn)題?