y=[4/(x^2)] +{9/[400-(x^2)]}

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y=[4/(x^2)] +{9/[400-(x^2)]}
讨论其单调性,并求出X值使y最小

$y=[4/(x^2)] +{9/[400-(x^2)]}$

柯西不等式呀~
（4/x²+9/(400-x²)）×(x²+(400-x))≥(2+3)²=25
y≥25/400=1/16
取等条件是（4/x²）/(9/(400-x²)) = (x²)/(400-x²)
x=4倍根号10

已知4x=9y求(1)x+y/y (2)y-x/2x 9y=3x-(x-1),4(x+y)-(2x+4)=8y 已知X.x+y.y-4x+2y+5=0,求4X.X-12X Y+9Y.y的值 y=(x-1)(x-2)(x-3)(x-4)(x-5)(x-6)(x-7)(x-8)(x-9)(x-10)的导数在x=1 y=[4/(x^2)] +{9/[400-(x^2)]} {3(x+y)-4(x-y)=4 {x+y/2 + x-y/6=1 x+y/2+x-y/3=6,4(x+y)-3(x-y)=-20 {(x+y)/2+(x-y)/3=6 4(x+y)-3(x-y)=-20 3(x+y)-4(x-y)=11,2(x-y)+5(x+y)=27 4(x+y)-3(x-y)=-20,2/x+y+3/x-y=6 (x+y)(x+2y)(x+3y)(x+4y)=-40 已知x/y=3/2求代数式4x-9y/2x+3y