已知x满足不等式?log1/2 x +3≤0求函数f(x)=(log2 x/4)(log2 x/2)的最值
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已知x满足不等式?log1/2 x +3≤0求函数f(x)=(log2 x/4)(log2 x/2)的最值
![已知x满足不等式?log1/2 x +3≤0求函数f(x)=(log2 x/4)(log2 x/2)的最值](/uploads/image/z/2165113-1-3.jpg?t=%E5%B7%B2%E7%9F%A5x%E6%BB%A1%E8%B6%B3%E4%B8%8D%E7%AD%89%E5%BC%8F%3Flog1%2F2+x+%2B3%E2%89%A40%E6%B1%82%E5%87%BD%E6%95%B0f%28x%29%3D%28log2+x%2F4%29%28log2+x%2F2%29%E7%9A%84%E6%9C%80%E5%80%BC)
log1/2 x +3≤0有
log1/2 x ≤-3=log1/2 8
∵log1/2(x)是减函数,有
∴ X≥8.
令t=log(2) x ( t≥3)
此时函数f(x)等价于:
y=(log2 x/4)(log2 x/2)
=(log2 x-log2 4)(log2 x-log2 2)
=(log2 x-2)(log2 x-1)
=(t-2)(t-1)
=t²-3t+2
=[t-(3/2)]²-(1/4)
其中,t≥3
则是当t=3,即log(2) x =3,x=8时y取得最小值2,
且f(x)无最大值
log1/2 x ≤-3=log1/2 8
∵log1/2(x)是减函数,有
∴ X≥8.
令t=log(2) x ( t≥3)
此时函数f(x)等价于:
y=(log2 x/4)(log2 x/2)
=(log2 x-log2 4)(log2 x-log2 2)
=(log2 x-2)(log2 x-1)
=(t-2)(t-1)
=t²-3t+2
=[t-(3/2)]²-(1/4)
其中,t≥3
则是当t=3,即log(2) x =3,x=8时y取得最小值2,
且f(x)无最大值
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