①(a^2+1)-4a^2 =1-3a^2
② 12/(m^2-9)-2/m-3
=12/(m+3)(m-3)-2/m-3
=12-2m-6/(m+3)(m-3)
=-2/(m+3)
③这是一种题型
解题模式就是令x/2=y/3=z/5=k
得x=2k
y=3k
z=5k
代入第二个式子得k=2
x=4
y=6
z=10