2*2^x + 2^(2x) - 80 = 0
Пусть t = 2^x, тогда
t^2 + 2t - 80 = 0
D = 4 + 4*80 = 324 = 18^2
t1 = ( - 2 + 18)/2 = 16/2 = 8;
t2 = ( - 2 - 18)/2 = - 20/2 = - 10 ==> не подходит
2^x = 8
2^x = 2^3
x = 3
q=25/125=1/5
b5= b1q^4=125*(1/5)^4=125/625=1/5
S5=-624/25*5/4=156/5=31,2