f(x)=lncosx
则f(0)=0
f'(x)=-tanx,f'(0)=0
f''(x)=-(secx)^2
f''(0)=-1
f'''(x)=-2(secx)^2·tanx
f'''(0)=0
f(4)(x)=-4(secx)^2·(tanx)^2-2(secx)^4
=-6(secx)^4+4(secx)^2
f(4)(0)=-2
f(5)(x)=-24(secx)^4·tanx+8(secx)^2·tanx
f(5)(0)=0
f(6)(x)=-96(secx)^4·(tanx)^2-24(secx)^6+16(secx)^2·(tanx)^2+8(secx)^4
f(6)(0)=-16
所以,6阶麦克劳林公式为
f(x)=-1/2·x^2-1/12·x^4-1/45·x^6+o(x^6)