2a.
(x+5)²-5x(2-x)=x²+10x+25-10x+5x²=6x²+25
2б.
16y+2(y-4)²=16y+2(y²-8y+16)=16y+2y²-16y+32=2y²+32
3a
.81-a²=9²-a²=(9-a)(9+a)
3б
5x²-5y²=5(x²-y²)=5(x-y)(x+y)
3в
3x²-6xy+3y²=3(x²-2xy+y²)=3(x-y)²
4
(a²+4a)-a²(a-2)(a+2)-4a²(2a-1)=(a²+4a)-a²(a²-4)-4a²(2a-1)=a²+8a³+16a²-a^4+4a²-8a³+4a²=-a^4+25a²