4a-1+b^2-4a^2=b^2-1+4a-4a^2=b^2-(4a^2-4a+1)=b^2-(2a-1)^2=(b+2a-1)(b-2a+1)
原式=b^2-(4a^2-4a+1) =b^2-(2a-1)^2 =(b+2a-1)(b-2a+1)
4a-1+b2-4a2=b2-(4a2-4a+1)=b2-(2a-1)2=(b+2a-1)(b-2a+1)
