原式=(a+c)^2+2b(a-c)
=(2/2m+4)^2+2(1/2m+2)(-2)
=4(1/2m+2)^2-4(1/2m+2)
=4(1/2m+2)(1/2m+2-1)
=4(1/2m+2)(1/2m-1)