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问题: 极值问题2(京)2008-12-20

极值问题2

解答:

f(x)在x=π/3处可导,所以极值点π/3是驻点,即f'(π/3)=0
f'(x)=acosx+3cos3x,f'(π/3)=a/2-3,所以a=6
f''(x)=-6sinx-9sin3x,f''(π/3)=-3√3<0
所以,a=6,f(x)在x=π/3处取得极大值f(π/3)=3√3