若圆锥底面圆的周长为8π,侧面展开图的圆心角为90°,则该圆锥的母线长为 .
1-a4=___________
-a2+b2=(a+b)(______)
x(a-1)(a-2)-y(1-a)(2-a)=____________.
3(y-x)2+2(x-y)=___________
-7ab+14a2-49ab2=-7a(________).