в) r5 – r10 + r15 r7 – r12 + r17.

$$ \frac{r^5 - r^{10} + r^{15}}{r^7 - r^{12} + r^{17}} = \frac{r^5(1 - r^5 + r^{10})}{r^7(1 - r^5 + r^{10})} = \frac{r^5}{r^7} = r^{5-7} = r^{-2} = \frac{1}{r^2} $$

Ответ: 1/r²