Đề bài

a) ${{x + y} \over {x - y}} + {{{x^2} - 4{y^2}} \over {{x^2} - {y^2}}} - {{x - 3y} \over {x + y}}$ ;

b) ${1 \over {2z - 3}} - {2 \over {3 - 2z}} + {{18} \over {9 - 4{z^2}}}$ ;

c) ${1 \over {{a^2} - 5a - 6}} - {a \over {a - 6}}$ ;

d) ${x \over {x + y}} + {4 \over {{x^2} + 3xy + 2{y^2}}} - {{3x} \over {x + 2y}}$ .