Question

等比数列an中，有a3*a11=4a7，数列bn是等差数列，且b7=a7，则b5+b9=

Answer 1

a3*a11=a7^2=4a7,所以a7=4 b7=a7=4 那么
则b5+b9=2b7=8

Answer 2

a3·a11=4a7
a1q^2*a1q^10=4a1q^6
a1^2q^12=4a1q^6
a1q^6=4=a7=b7
b5+b9
=b1+4+b1+8d
=2b1+12d
=2(b1+6d)
=2b7
=2*4
=8