Question

在等比数列{an}中 已知a1乘a2等于324 a3乘a4等于36 则a5乘a6的值为多少啊! 帮一下忙 急啊!

Answer 1

等比数列{an}中， a1a2=a1*a1q=a1�0�5q

a3a4=(a1q�0�5)*(a1q^3)=a1�0�5q^5

a5a6=(a1q^4)(a1q^5)=a1�0�5q^9 因为 a1�0�5q xa1�巧郑腔0�5q^9=(a1�0�5q^5 )
即（a1a2）x(a5a6)=(a3a4)�0�5

所以(a5a6)=(a3a4)�0�丛棚5/（a1a2）=36�0�5/孝衫324=4
所以 a5a6=4

Answer 2

解：设公比是友氏d,则州乱a2=a1d,a3=a1d^2,a4=a1d^3,∴a1a2=a1^2d=324,a3a4=a1^2d^5=36.∴324d^4=36,d^4=36/324∴d^8=(36/324)^2,∴a5a6=a1d^4a1d^5=a1^2dd^8=324*(36/册告档324)^2=4.∴a5a6=4.