Given:
a denotes first term and r denotes the common ratio.
[tex]a_{10}=1\colon a_{12}=\frac{1}{25}[/tex][tex]a_n=ar^{n-1}[/tex][tex]a_{10}=ar^{10-1}[/tex][tex]1=ar^9\ldots.\text{ (1) }[/tex][tex]a_{12}=ar^{12-1}[/tex][tex]\frac{1}{25}=ar^{11}\ldots.(2)[/tex]Divide the equation (2) by (1)
[tex]\frac{\frac{1}{25}}{1}=\frac{ar^{11}}{ar^9}[/tex][tex]\frac{1}{25}=r^2[/tex][tex]r=\pm\frac{1}{5}[/tex][tex]\text{If r=}\frac{1}{5}[/tex][tex]1=a(\frac{1}{5})^9[/tex][tex]a=1953125[/tex][tex]\text{If r=-}\frac{1}{5}[/tex][tex]1=a(-\frac{1}{5})^9[/tex][tex]a=-1953125[/tex][tex]a=-1953125\text{ ; r = -}\frac{1}{5}[/tex][tex]a=1953125\text{ ; r = }\frac{1}{5}[/tex]