the first term of a geometric series is 1,the nth term is 128 and the sum of the n terms is 256 find the common ratio and the number of terms.

Guest Jul 4, 2017

#1**0 **

lets call the ratio q, ok?

the sum of the first term+the second term+.....+the nth term=a_{1}*(q^{n}-1)/(q-1). a_{1}=1, so the sum=(q^{n}-1)/(q-1). we know the nth term is 128, so q^{n-1}=128, so q^{n}=128*q

128*q-1/q-1=256

128*q-1=256*q-256

-1=128*q-256

128*q=255

q=255/128

the series is impossible

Guest Jul 4, 2017