==f（x）=3/4x^4+2/3ax^3+2x^2+b ,
由f'(x)=3x^3+2ax^2+4x=0得
x1=0,或3x^2+2ax+4=0,
f（x）仅在x=0处有极值,
∴Δ＝4a^2-48