A=x+2x2-3x3,B=3x3-3x2-x-4,其中x=-2/3,求2A-(-A-B)的值x不是乘号哦·~
问题描述：
A=x+2x2-3x3,B=3x3-3x2-x-4,其中x=-2/3,求2A-(-A-B)的值
x不是乘号哦·~
答
2A-(-A-B)=3A+B
A=x+2x2-3x3,B=3x3-3x2-x-4A+B=-x2-42A=2x+2x2-3x3则 3A+B=2x+x2-3x3-4=-4/3+4/9+8/9-4=-12/9+13/9-4=1/9-4=-3又8/9
答
解
2A-(-A-B)
=3A+B
=3(x+2x²-3x³)+(3x³-3x²-x-4)
=(-9x³+3x³)+(3x-x)+(6x²-3x²)-4
=-6x³+3x²+2x-4
=-6×(-2/3)³+3×(-2/3)²+2(-2/3)-4
=16/9+12/9-4/3-4
=16/9-36/9
=-20/9