已知数列{an}是等差数列,{bn}是等比数列,且a1=b1=2,b4=54,a1+a2+a3=b2+b3,求数
问题描述:
已知数列{an}是等差数列,{bn}是等比数列,且a1=b1=2,b4=54,a1+a2+a3=b2+b3,求数
答
a=b=2,p=6,q=3;
答
b4 = b1* q^3,所以公比q = 3,b2 = b1*3 = 6,b3 = b1*3*3 = 18.a1+a2+a3 = 3*a2 = 6+18,所以
a2 = 8,所以公差d = a2 -a1 = 6.a3 = 8+6 = 14.