Question

{an}是等比数列若a1+a2+a3=26 ,a4-a1=52,求an

Answer 1

a1+a2+a3=26,相当于a1(1+q+q*q)=26,a4-a1=52相当于a1（q*q*q-1）=a1（q-1）（q*q+q+1）=52,上面两式相除得,q-1=2,所以q=3,带入就得a1=2,an=2*3n-1.