C-13-Q108mediumsingle_mcqWhich term of the series 128+64+32+⋯128 + 64 + 32 + \cdots128+64+32+⋯ is 12\dfrac{1}{2}21?a999thb888thc101010thd777thব্যাখ্যাSet 128(12)n−1=12128\left(\tfrac12\right)^{n-1}=\tfrac12128(21)n−1=21, so (12)n−1=1256=(12)8\left(\tfrac12\right)^{n-1}=\dfrac{1}{256}=\left(\tfrac12\right)^{8}(21)n−1=2561=(21)8, giving n−1=8n-1=8n−1=8, hence n=9n=9n=9. So it is the 999th term.