[กิตติ]

ผมทำแบบนี้ครับ
$9b^2=8ac$....(1)
$\frac{a+c}{ac}=\frac{2}{b} $.....(2)
จาก(1)....$ac=\frac{9b^2}{8} $.....นำไปแทนใน(2)
$\frac{8}{9}\left(\,\frac{a+c}{b^2} \right)=\frac{2}{b} $
$\frac{a+c}{b}=\frac{9}{4} $....(3)

$\frac{a}{c}+\frac{c}{a} =M $
$\frac{a}{c}+1+\frac{c}{a}+1 =M+2$
$(a+c)\left(\,\frac{1}{c}+ \frac{1}{a} \right)=M+2 $
$(a+c)(\frac{2}{b})=M+2$
$\frac{9}{2}=M+2 $
$M=\frac{5}{2} $