18.设α,β是方程5x2+10x+1=0的两个根,则2α·2β=__________,(2α)β=__________.
19.若,求的值.(注:(a+b)3=a3+3a2b+3ab2+b3)
20.已知a,b是方程x2-6x+4=0的两根,且a>b>0,求的值.