2.p^2-p+1/4=(p-1/2)^23.-x^2-y^2=y^2-x^2=(y+x)(y-x)
2，3能因式分解2.p^2-p+1/4=p^2-2*1/2*p+(1/2)^2=(p-1/2)^23.-x^2+y^2=(y+x)(y-x)
