r, s, t eqatigisitsiniut peqataasanut x^3 - 20x^2 + 18x - 7 = 0. r/(1/r+st) + s/(1/s+tr) + t/(1/t+rs) iliorneq: 
r/(1/r+st) + s/(1/s+tr) + t/(1/t+rs) = (r^2/(1+rst)) + (s^2/(1+rst)) + (t^2/(1+rst)) 
= (r^2+s^2+t^2)/(1+rst) 
= ((r+s+t)^2 - 2(rs+rt+st))/(1+rst) 
= ((20)^2 - 2(18))/(1+7) 
= (400 - 36)/8 
= 364/8 
= 91/2
ilusilersorneq: 

r/(1/r+st) = (r^2)/(1+rst) = (r^2)/(1+7) = (r^2)/8, 

tuneqarsinnaanera Vieta's formulat qallunaat. Aamma qalipaat, taama: 

r/(1/r+st) + s/(1/s+tr) + t/(1/t+rs) = (r^2+s^2+t^2)/8, 

tassaasoq: 

((r+s+t)^2 - 2(rs+st+tr))/8 = (20^2 - 2*18)/8 = 91/2.