急！！七年级下册数学题，只有三题！！谢谢各位！！！

Question

如题所述

Answer 1

1、(1)(x-2y)^2=0，x=2y；
（x-y)/(x+y)=(2y-y)/(2y+y)=1/3.
(2)x=2y/3；
(7x^2-y^2)/(x^2-2xy+3y^2)
=(28y^2/9-y^2)/(x+y)(x-3y)
=(19y^2/9)/(5y/3)(-7y/3)
=-19/35
2、(1)=(a^4b^2)(a^2b^6)/(4a^2b^6)(9a^6b^2)
=b^2/36a^2
(2)=(x+y+z)(x+y-z)/(x-z+y)(x-z-y)
=(x+y+z)/(x-y-z)
3、(1)=[1/(x-1)^2-1/(x+1)^2]÷[2x/(x+1)(x-1)]
=4x/(x-1)^2(x+1)^2×(x+1)(x-1)/2x
=2/(x^2-1)
(2)=[a(b-c)-b(a-c)+c(a-b)]/(a-b)(a-c)(b-c)
=(ab-ac-ab+bc+ac-bc)/(a-b)(a-c)(b-c)
=0
(3)=1/x-1/(x+1)+1/(x+1)-1/(x+2)+1/(x+2)-1/(x+3)+1/(x+3)-1/(x+4)
=1/x-1/(x+4)
=3/x(x+4)