P(−n)=−mn3+mn2−mn+n=0 n=0
−mn2+mn−m+1=0
m=n2−n+11 Let m=f(n)=n2−n+11,mn=0
n2−n+1=n2−n+41+43
=(n−21)2+43>0,n∈R
f′(n)=−(n2−n+1)22n−1 Find the critical number(s)
f′(n)=0=>−(n2−n+1)22n−1=0=>n=21 If n∈(−∞,0)∪(0,21), then f′(n)>0,f(n) increases.
If n∈(21,∞), then f′(n)<0,f(n) decreases.
The function f(n) has a local maximum at n=21.
f(21)=(21)2−21+11=34
n→0−limf(n)=n→0+limf(n)=1
n→−∞limf(n)=n→∞limf(n)=0 Then m∈(0,34]
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