а^3-(a+1)^3+3a^2 =а^3+3a^2-(а^3+3a^2+3а+1)=3а+1
а^3-(a+1)^3+3a^2= a^3 - (a^3+3a^2 * 1 +3a1^2+ 1^3)+3a^2= a^3 - a^3 -3a^2 -3a -1 + 3a^2= -3a-1= - (3a+1)
а^3-(a+1)^3+3a^2 =а^3+3a^2-(а^3+3a^2+3а+1)=3а+1
а^3-(a+1)^3+3a^2= a^3 - (a^3+3a^2 * 1 +3a1^2+ 1^3)+3a^2= a^3 - a^3 -3a^2 -3a -1 + 3a^2= -3a-1= - (3a+1)