По свойству геометрической прогрессии:
а(4) =√ (54*6 ) = ±18 и g = ±18 / 54 = ±(1/3)
Возможны два случая:
а) g = 1/3 , тогда а1 = 54 : (1/3)²= 54*9 = 486
S(6) = 486( 1- 1/729)/ (1-1/3) = 486*728*3/ (729*2) = 728
б) g = - 1/3 тогда а1 = 54 : (-1/3)²= 54*9 = 486
S(6) = 486( 1- 1/729)/ (1+1/3) = 486*728*3/ (729*4) = 364