（1）x²-4xy+4y²-10x+20y+25 =x²-2×2xy+（2y）²-（10x-20y）+25=(x-2y)²-2×5（x-2y)+5²=(x-2y-5)²(2)(n+1)(n+2)(n+3)(n+4)+1=(n+1)(n+4)(n+2)(n+3)+1=(n²+5n+4)(n²+...