12个小朋友在一起,yy2的值分别有怎样的变化?后回答: 根据所填表格,则这个多项式为 a.4x2+6x+2 b.-4x+2 c.-6x+2 d.4x+28.下列各式运算 (1)-(-a-b)=a-b; (2)5x-(2x-1)-x2=5x-2x-1+x2; (3)3xy- (xy-y2)=3xy- xy+y2; (4)(a3+b3)-3(2a3-3b3)=a3+b3-6a3+9b3 其中去括号不正确的有 a.(1)(2) b.(1)(2)(3) c.(2)(3)(4) d.(1)(2)(3)(4)9.∠