如图,20题和21题,
来源:学生作业帮 编辑:搜狗做题网作业帮 分类:数学作业 时间:2024/07/16 10:25:33
如图,20题和21题,
![](http://img.wesiedu.com/upload/7/5d/75d6afb12382ff749697596cd20adb6d.jpg)
![](http://img.wesiedu.com/upload/7/5d/75d6afb12382ff749697596cd20adb6d.jpg)
![如图,20题和21题,](/uploads/image/z/6588067-67-7.jpg?t=%E5%A6%82%E5%9B%BE%2C20%E9%A2%98%E5%92%8C21%E9%A2%98%2C)
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