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5,6选几呀,过程,谢谢
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5
水平方向长xm,则垂直方向长(8-3x)/2m;
不计边框宽度,
S=x(8-3x)/2
=(1/2)(8x-3x²)
=(1/2)[-3(x²-8x/3+16/9-16/9)]
=(1/2)[-3(x-4/3)²+16/3]
=8/3-(3/2)(x-4/3)²
x=4/3m时,S最大值=8/3m²;
选择B
6
AD=xm,AB=80-2xm,
S=AD*AB
=x(80-2x)
=80x-2x²
=-2(x²-40x+400-400)
=-2(x-20)²+800
x=20m时,S有最大值800m².[此时AB=80-2x=80-40=40<45,满足题目要求]
水平方向长xm,则垂直方向长(8-3x)/2m;
不计边框宽度,
S=x(8-3x)/2
=(1/2)(8x-3x²)
=(1/2)[-3(x²-8x/3+16/9-16/9)]
=(1/2)[-3(x-4/3)²+16/3]
=8/3-(3/2)(x-4/3)²
x=4/3m时,S最大值=8/3m²;
选择B
6
AD=xm,AB=80-2xm,
S=AD*AB
=x(80-2x)
=80x-2x²
=-2(x²-40x+400-400)
=-2(x-20)²+800
x=20m时,S有最大值800m².[此时AB=80-2x=80-40=40<45,满足题目要求]
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