∵{an},{bn}是两个等差数列 Sn/Tn=（7n+2）/（n+3）∴[n/2(a1+an)]/[n/2(b1+bn)]=(7n+2)/(n+3)∴(a1+an)/(b1+bn)=(7n+2)/(n+3)∴a5/b5=(2a5)/(2b5)=(a1+a9)/(b1+b9)=(7×9+2)/(9+3)=65/12 求a5/a6缺条件：
