If the first, second and last terms of an $A.P.$ be $a,\;b,\;2a$ respectively, then its sum will be
If the first, second and last terms of an $A.P.$ be $a,\;b,\;2a$ respectively, then its sum will be
- A
$\frac{{ab}}{{b - a}}$
- B
$\frac{{ab}}{{2(b - a)}}$
- C
$\frac{{3ab}}{{2(b - a)}}$
- D
$\frac{{3ab}}{{4(b - a)}}$
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