已知三个非负实数a,b,c满足：3a+2b+c=5和2a+b-3c=1,若m=3a+b-7c,则m的最小值是多少?
把两个未知数用一个未知数表示3a+2b=5-c(1)2a+b=1+3c(2)消掉b,（2）乘2减去（1）得a=7c-3同理b=7-11c因为a和b为非负实数,所以a≥0,b≥0所以7-3c≥0,7-11c≥0c≥3/7 c≤7/11 综合得3/7≤c≤7/11 M=3(7c-3)+(7-11c)-7c=3c-2要M最小,故c最小.所以当c=3/7时,M最小.M=3*（3/7）-2=-5/7（负七分之五）
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偶想不到简单的方法..只能用最麻烦的方法了..3式联立，的a=(5+7m)\3
b=-1\3*(1+11m)
由a,b,c大于等于0，得m大于等于-5\7小于等于-1\11
所以最小值为-5\7
3a+2b+c=5(1); 2a+b-3c=1(2). 由以上两式可得：a=7c-3≥0,c≥3/7;b=7-11c≥0,c≤7/11. 即：3/7≤c≤7/11; 故m=3a+b-7c=3(7c-3)+(7-11c)-7c=3c-2 m最小值为3*(7/11)-2=-1/11
扫描下载二维码怀孕周径长7.3C M，宽7.9cm，前后径：7.1，孕囊4.3*2.6C M，芽长1.6cm是男_百度知道
怀孕周径长7.3C M，宽7.9cm，前后径：7.1，孕囊4.3*2.6C M，芽长1.6cm是男
9cm怀孕周径长7.3C M，前后径.6C M：7.1，孕囊4.3*2，芽长1，宽7
不出的，你这个宝宝大小大约也就58-60天左右，民间所谓的孕囊长的圆的，才能判断，而且是看到生殖器官才能判断，没有科学依据的，看到生殖器官了，不准的，B超要16周以后才能看到
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出门在外也不愁函数y=x 2 -4x+3化成y=（x+m） 2 +k的形式是（　　）
A．y=（x-2） 2 -1
B．y=（x+2） 2 -1
C．y=（x-2） 2 +7
D．y=（x+2） 2 +7
y=x 2 -4x+3，=（x 2 -4x+4）-4+3，=（x-2） 2 -1．故选A．
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