令f'(x)=3x^2-10x+3=0,可得x=1/3或3
f(x)单调区间：
(-∞,1/3]单调增；(1/3,3)单调减；(3,+∞)增
令f''(x)=6x-10＝0,x=5/3,此时y=20/27,即拐点为(5/3,20/27)
故在区间(-∞,5/3)函数图像上凸；在区间(5/3,+∞)图像下凹.