解;设等差数列中a1=a-4d ,a5=a a7=a+2d
又数列a1,a5,a7为等比数列
则(a5)^2=a1*a7
即a^2=(a+2d)(a-4d)
a^2=a^2-4ad+2ad-8d^2
a=-4d
a1+a5+a17/ a2+a6+a18
=(a-4d+a+a+12d)/(a-3d+a+d+a+13d)
=(3a+8d)/(3a+11d)
=(-12d+8d)/(-12d+11d)
=4
解;设等差数列中a1=a-4d ,a5=a a7=a+2d
又数列a1,a5,a7为等比数列
则(a5)^2=a1*a7
即a^2=(a+2d)(a-4d)
a^2=a^2-4ad+2ad-8d^2
a=-4d
a1+a5+a17/ a2+a6+a18
=(a-4d+a+a+12d)/(a-3d+a+d+a+13d)
=(3a+8d)/(3a+11d)
=(-12d+8d)/(-12d+11d)
=4