(a) $S_{3rd} = 10 + \frac{10}{2}(2 \times 3 - 1) = 35$ $S_{2nd} = 10 + \frac{10}{2}(2 \times 2 - 1) = 25$ $\therefore\ \frac{S_{3rd}}{S_{2nd}} = \frac{35}{25}\ ie = \frac{7}{5}$
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