已知A=2x2+3xy-2x-1，B=-x2+xy-1，求3A+6B

1. 已知A=2x2+3xy-2x-1，B=-x2+xy-1，求3A+6B

∵A=2x2+3xy-2x-1，B=-x2+xy-1，∴3A+6B=3（2x2+3xy-2x-1）+6（-x2+xy-1）=6x2+9xy-6x-3-6x2+6xy-6=15xy-6x-9．

2. y=ax²+bx+c,当x=1时y=3,当x=-1时,y=5求2ab+2bc-3b平方

应该是等式y=ax²+bx+c吧?
  如果是,则将x=-1,y=0；x=2,y=3；x=5,y=60.分别代入,得
  {a-b+c=0
  4a+2b+c=3
  25a+5b+c=60
  解得:{a=3
  b=2
  c=-1

3. 若A=x-2y,B=3x-y,求A+2B

A+2B
  =x-2y+6x-2y
  =7x-4y

4. a=3x2-4xy+2y2,b=x2+2xy-5y2求:1、a+b2、a-b3、若2a-b+c=0

∵2A-B+C=0
  ∴C=B-2A
  =(x²+2xy-5y²)-2(3x²-4xy+2y²)
  =x²+2xy-5y²-6x²+8xy-4y²
  =-5x²+10xy-10y²
  =-5×(-2)²+10×(-2)×(-1)-10×(-1)²
  =-20+20-10
  =-10

5. a*x^3+b*x^2+c*x=y ,已知x1,y1,x2,y2,x3,y3,求a,b,c

a=y1/[(x1-x2)(x1-x3)]+y2/[(x2-x1)(x2-x3)]+y3/[(x3-x1)(x3-x2)]
b=-y1(x2+x3)/[(x1-x2)(x1-x3)]-y2(x1+x3)/[(x2-x1)(x2-x3)]-y3(x1+x2)/[(x3-x1)(x3-x2)]
c=y1x2x3/[(x1-x2)(x1-x3)]+y2x1x3/[(x2-x1)(x2-x3)]+y3x1x2/[(x3-x1)(x3-x2)]
都是轮换对称的式子。

6. 若2ax+1b3+3a3x-1by=5ax+1b3，则x+y=______

∵2ax+1b3+3a3x-1by=5ax+1b3，∴x+1=3x-1，y=3，∴x=1，y=3，∴x+y=1+3=4．故答案为4．

7. 已知：|x?a|＜c4，|y?b|＜c6，求证：|2x?3y?2a+3b|＜c

证明：|2x-3y-2a+3b|=|2（x-a）-3（y-b）|≤|2（x-a）|+|3（y-b）|=2|x-a|+3|y-b|＜2×c4+3×c6＝c．

8. a=x+2x+2y-1，b=3+y-2xy+2x，若a+b+c=0，求c

A+B=2+3x的三次方+2x的平方+3Y的三次方-2xy 所以C=-2-3x的三次方-2x的平方-3Y的三次方+2xy