(1-16a的4次)除以(4a的2次+1)除以(2a+1)
=(1-16a^4)/(4a^2+1)/(2a+1)
=(1+4a^2)(1-4a^2)/(1+4a^2)/(1+2a)
=(1-4a^2)/(1+2a)
=(1+2a)(1-2a)/(1+2a)
=1-2a