6a
1)a²-(y²+x²-2xy)=a²-(y-x)²=(a-y+x)(a+y-x)
2)(a-y+x)(a+y-x) :(a+y-x)/(a+y+x)=(a-y+x)(a+y-x) *(a+y+x)/(a+y-x)=
=(a-y+x)(a+y+x) или (a+x)²-y²
7a
1)1/3(1+3p+9p²)+p(3p-1)(3p+1)/(3p-1)(9p²+3p+1)=
=1/3(1+3p+9p²)+(3p²+p)/(9p²+3p+1)=(1+9p²+3p)/(9p²+3p+1)=1/3
2)1/3:1/(3p+1)²=(3p²+1)/3