Có ai biết làm bài này ko giúp với

7.12

x2–3x+2+4x3–x2+x–1 =4x3+(x2–x2)+(−3x+x)+(2−1) =4x3–2x+1

7.13

7.14

A+B=6x4–4x3+x−13+(−3x4–2x3–5x2+x+23)      

=6x4–4x3+x−13−3x4–2x3–5x2+x+23     

 =(6x4–3x4)+(−4x3–2x3)–5x2+(x+x)+(−13+23)    

  =3x4−6x3–5x2+2x+13 A−B=6x4–4x3+x−13−(−3x4–2x3–5x2+x+23)     

 =6x4–4x3+x−13+3x4+2x3+5x2−x−23      

=(6x4+3x4)+(−4x3+2x3)+5x2+(x−x)+(−13−23)    

  =9x4−2x3+5x2−1