(d) This progression is an arithmetico-geometric series. $\therefore\ S_{\infty} = \frac{a}{1 - r} + \frac{dr}{(1 - r)^{2}}$ $= \frac{1}{1 - 1/2} + \frac{2}{(1 - 1/2)^{2}}$ $= \ \frac{2}{1/2} + \frac{2}{1/4}$ $= 4 + 8 = 12$
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Ph.D. in Physics with 19+ years of teaching experience.