<prompt> Acha r, s, na t ziwe mizizi ya mlinganyo x3 - 20x2 18x - 7 0. Tafuta thamani ya fracrfrac1 rst fracsfrac1str fractfrac1trs.  <response>: Kumbuka kwamba fracrfrac1rst fracr21rst fracr217 frac r28,tangu rst7 kulingana na fomula za Vieta. Kwa hesabu sawa, tunapata fracrfrac1rst fracsfrac1str fract frac1trs fracr2s2t28,ambayo ni sawa na fracrst2 - 2rssttr8frac202 - 2cdot 188 frac912. 