**Solution:**

The first term of the G.P is *a* and the last term is *b*.

Therefore, the G.P. is *a*, *ar*, *ar*^{2}, *ar*^{3}, … *ar*^{n–1}, where *r* is the common ratio.

*b* = *ar*^{n–1} … (1)

*P* = Product of *n* terms

= (*a*) (*ar*) (*ar*^{2}) … (*ar*^{n–1})

= (*a* × *a* ×…*a*) (*r* × *r*^{2} × …*r*^{n–1})

= *a**n* *r* ^{1 + 2 +…(n–1) }… (2)

Here, 1, 2, …(*n* – 1) is an A.P.

∴1 + 2 + ……….+ (n – 1)