Question

化简求值:(x-2)(x+3)+3(x+1)(x-1)-(2x+1)(2x-3),其中x=4/5

Answer 1

:(x-2)(x+3)+3(x+1)(x-1)-(2x+1)(2x-3)
=x²+3x-2x-6+3x²-3-4x²+3x-2x+3
=2x-6
=2×4／5-6
=-4.4

追问

看不懂+3x-2x-6+3x²-3-4x²+3x-2x+3这一步，能更详细一点吗？？

追答

:(x-2)(x+3)+3(x+1)(x-1)-(2x+1)(2x-3)
=x²+3x-2x-6+3x²-3-4x²+3x-2x+3
=﹙1+3-4）x²+(3-2+3-2)x+(-6-3+3)
=2x-6
=2×4／5-6
=-4.4

追问

第二步还是看不懂，你现在又给我来个第三步，更看不懂，你能把第二步说的更详细一点吗？？

追答

（a+b)(c+d)=ac+ad+bc+bd
代入就是

Answer 2

(x-2)(x+3)
=x²+3x-2x-6
3(x+1)(x-1)
=3x²-3
(2x+1)(2x-3)=4x²-6x+2x-3
:(x-2)(x+3)+3(x+1)(x-1)-(2x+1)(2x-3)=x²+x-6+3x²-3-4x²+4x+3=5x-6=-2

Answer 3

解:(x-2)(x+3)+3(x+1)(x-1)-(2x+1)(2x-3)
=x²+x-6+3x²-3-4x²+4x+3
=5x+6
代入x=4/5 解得原式=10！
望采纳！！