B13=sqrt(b12*b14)=sqrt(3^15*3^17)=sqrt(3^32)=3^16
b13=b1*q^12, b12=b1*q^11
q=b13/b12=3^16/3^15=3
b1=b12/q^11=3^15/3^11=3^4
Решение :
n = ( an - a1 )/d + 1; n = ( 400 - 9 )/9 + 1 = 44;a44 = a1 + d * 43;a44 = 9 + 9 * 43 = 396;S44 = 0,5 * ( 2 * a1 + d * ( n - 1))* n;S44 = 0,5 * ( 18 + 9 * 43 ) * 44;S44 = 8.910.
Вроде так, ..................................................................................