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a)2,4,8,16... a_1=2, q=4:2=2 S_6=\frac{a_1(1- q^6}{1-q} S_6=\frac{2(1- 2^6}{1-2} S_6=\frac{2(1-64}{-1} S_6=\frac{2\cdot (-63)}{-1} S_6=\frac{-126}{-1} S_6=126 b)1,-2,4,-8... a_1=1, q=-2:1=-2 S_6=\frac{a_1(1- q^6}{1-q} S_6=\frac{1(1- (-2)^6}{1+2} S_6=\frac{1(1-64}{3} S_6=\frac{-63}{3} S_6=-21 c)1,\frac{1}{2}, \frac{1}{4}, \frac{1}{8} a_1=1, q=\frac{1}{2}:1=\frac{1}{2} S_6=\frac{a_1(1- q^6}{1-q} S_6=\frac{1(1-(\frac{1}{2})^6}{1-\frac{1}{2}} S_6=\frac{1(1-\frac{1}{64}}{\frac{1}{2}} S_6=\frac{\frac{63}{64}}{\frac{1}{2}} S_6=\frac{63}{32}