Question

Can someone please explain me this

Answer 1

1st way

To solve this we need to know ratio.

$- \frac{1}{5} \div \frac{1}{5} = - 1$

Now we can see that odd numbers of geometric sequence are positive and even numbers are negative. 91 - odd number, so it will be positive. If ratio is equal to -1, than 91st number is

$\frac{1}{5}$

2nd way

Firstly, we need to find ratio.

$- \frac{1}{5} \div \frac{1}{5} = - 1$

Secondly, we need to use this formula

$b_n=b_1 \times q^{n-1}$

$b_{91}= \frac{1}{5} \times ( - 1)^{91-1} = \frac{1}{5}$