Question

If a, b, c are in H.P and ab + bc + ca $=$15, then ca $=$

• A. $5$
• B. $7$
• C. $9$
• D. $10$

Answer 1

The correct option is A $5$

Lets divide the whole equation by $abc$ we get $\frac{1}{a}+\frac{1}{b}+\frac{1}{c}=\frac{15}{abc}$
Now we know $\frac{1}{a}+\frac{1}{c}=\frac{2}{b}\left[HP\right]$
$\frac{3}{b}=\frac{15}{abc}$
$ac=5$
Option A