y'=12x^3-8x,y"=36x^2-8令y"=0,则x=±(1/3)√2x<-(1/3)√2时,凹-(1/3)√21/3)√2-------f(1/3)√2)=f(-1/3)√2)=7/27,拐点:(-(1/3)√2, 7/27)及（(1/3)√2，7/27,）
