神马作文网如图,20题和21题,
来源：学生作业帮 编辑：神马作文网作业帮 分类：数学作业 时间：2024/11/12 22:57:44
如图,20题和21题,
21.选B
△ABC的最大面积为1/2ab
∵1/2(a²+b²)-1/2ab
=1/2(a²-ab+b²)
=1/2(a-1/2b)²+3/4b²＞0
∴1/2(a²+b²)＞1/2ab
∴选B
22.选D
∵x1,x2是方程x²+x-3=0的两个根
∴x1+x2=-1,x1²=3-x1,x2²=3-x2
∴x1³-4x2²+19
=x1(3-x1)-4(3-x2)+19
=3x1-x1²-12+4x2+19
=3x1-(3-x1)-12+4x2+19
=4(x1+x2)-3-12+19
=-4-2-12+19
=0
△ABC的最大面积为1/2ab
∵1/2(a²+b²)-1/2ab
=1/2(a²-ab+b²)
=1/2(a-1/2b)²+3/4b²＞0
∴1/2(a²+b²)＞1/2ab
∴选B
22.选D
∵x1,x2是方程x²+x-3=0的两个根
∴x1+x2=-1,x1²=3-x1,x2²=3-x2
∴x1³-4x2²+19
=x1(3-x1)-4(3-x2)+19
=3x1-x1²-12+4x2+19
=3x1-(3-x1)-12+4x2+19
=4(x1+x2)-3-12+19
=-4-2-12+19
=0