y=(x-1)x^2/3的极值

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y=(x-1)x^2/3的极值

y=(x-1)x^2/3的极值
y=(x-1)x^2/3的极值

y=(x-1)x^2/3的极值
其导数y'=x^(2/3)+(2/3)·x^(-1/3)(x-1)=(5/3)·x^(2/3) - (2/3)·x^(-1/3)
令y'=0,则(5/3)·x^(2/3) - (2/3)·x^(-1/3)=0
5·x^(2/3) - 2·x^(-1/3)=0
两边乘x^(1/3)得
5x - 2 =0
x= 2/5
y''=(10/9)·x^(-1/3) + (2/9)·x^(-4/3)
则y''(2/5)恒>0.
说明y(2/5)是极小值,为 (-3/5)·(2/5)^(2/3)= -3·2^(2/3) /5^(5/3)
当x=0时,y''=(10/9)·x^(-1/3) + (2/9)·x^(-4/3)=0,是拐点,不是极值点

求导法求极值:
y'=x^2-2/3x=0
x=0或者x=2/3
f(0)=0
f(2/3)=-4/81;为两个极值

求导得x^2-x*2/3,令其等于0解得x=0或x=2/3,代入原式得x=2/3时取极小值-4/81;x=0时取极大值0。