Variant training 2- 1: b,

Variant training 3- 1: I can't see clearly

Variant training 3-2:( 1) Original formula =-x(xy-y? )? ;

(2) The original formula =(a? -B? )(m-n)=(a+b)(a-b)(m-n)；

(3) The original formula =(x+y-4xy)(x+y).