Consider a geometric sequence that has a common ratio 7 and

Consider a geometric sequence that has a common ratio 7 and second term equal to 28. Find a5 and S12

common ratio r = 7

Formula for nth term of geometric series is given as : an = a1r^(n-1)

Given a2= 28 .Find a1 from this information:

a2 = a1r^(2-1)

a2 = a1*r ----> 28 = a1*7

a1 = 4

a5 : a5 = a1r^(5-1) = 4*7^(5-1) = 4*7^4 = 9604

Sum of 12 terms = a1(r^n -1)/(r-1) = 4( 7^12 -1)/( 7-1) = 9227524800