求xy的最大值 求xy最大值
x+y=300\u6c42xy\u7684\u6700\u5927\u503c\uff0cxy\u7b49\u4e8e\u591a\u5c11\u89e3\uff1ax+y=300
\u8981\u6c42xy\u7684\u6700\u5927\u503c\uff0c\u663e\u7136\u4e0d\u53ef\u80fd\u4e00\u6b63\u4e00\u8d1f\uff0c\u5426\u5219xy0\u4e14y>0
\u6839\u636e\u5747\u503c\u4e0d\u7b49\u5f0f\u6709
(x+y)/2\u2265\u221a(xy)\u5f97
\u221a(xy)\u2264150
xy\u226422500
\u5f53\u7136\u5982\u679c\u6ca1\u5b66\u8fc7\u5747\u503c\u4e0d\u7b49\u5f0f\uff0c\u5229\u7528\u4e8c\u6b21\u51fd\u6570\u6c42\u6700\u503c\u5f97\u529e\u6cd5\u4e5f\u53ef\u594f\u6548\uff1a
x+y=300\u5f97y=300-x\u4ee3\u5165\u5f97
xy=x(300-x)=-x²+300x=-(x-150)²+22500\u226522500
\u5f53\u4e14\u4ec5\u5f53x=150,y=150\u65f6\u53d6\u6700\u5927\u503c\u3002
10
当y=1/2时能取的最大值 所以Z即xy最大值为1/2(x>0,0<y<1)
应该是1/2.
2=x+2y≥2√x*2y=2√2xy
2≥2√2 *√xy
√xy≤√2 /2
xy≤1/2
符号很难打啊 你慢慢体会吧
X=2-2y 带入xy中可得 (2-2y)y
配方得 -2(y-1/2)^2+1/2 所以当y=1/2时 x=1 xy 有最大值1/2
xy=1/2×x×2y≤1/2[(x+2y)/2]^2=0.5
当且仅当x=2y=1时取等
xy=(2-2y)*y=2y-2y*y
学过二元一次方程没?求它的最值会么?
绛旓細xy=(2-2y)y=-2y^2+2y=-2(y-1/2)^2+1/2 鏁呭綋y=1/2鏃,鍙栨渶澶у兼槸1/2.
绛旓細鍏蜂綋姹傛硶锛屽鍥炬墍绀
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