f(x)二阶可导,f(π)=0,f''(π)>0,x=π是f(x)的极值点,g(x)=f(x)cosx,则

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f(x)二阶可导,f(π)=0,f''(π)>0,x=π是f(x)的极值点,g(x)=f(x)cosx,则

f(x)二阶可导,f(π)=0,f''(π)>0,x=π是f(x)的极值点,g(x)=f(x)cosx,则
f(x)二阶可导,f(π)=0,f''(π)>0,x=π是f(x)的极值点,g(x)=f(x)cosx,则

f(x)二阶可导,f(π)=0,f''(π)>0,x=π是f(x)的极值点,g(x)=f(x)cosx,则
极值点都是驻点,因此f'(π)=0.
g'(x)=f'(x)cosx-f(x)sinx,g'(π)=f'(π)cosπ-f(π)sinπ=0;
g''(x)=f''(x)cosx-2f'(x)sinx-f(x)cosx,
g''(π)=f''(π)cosπ-2f'(π)sinπ-f(π)cosπ
=-f''(π)