Question

Find ula"> in a geometric series for which $S_{n}$ = 189, r = $\frac{1}{2}$, and $a_{n}$ = 3.

Answer 1

Answer:

$\displaystyle a_{1} = 108$

Step-by-step explanation:

we are given

the sum,common difference and nth term of a geometric sequence

we want to figure out the first term

recall geometric sequence

$\displaystyle S_{ \text{n}} = \frac{ a_{1}(1 - {r}^{n} )}{1 - r}$

we are given that

• $S_n=189$
• $r=\dfrac{1}{2}$
• $n=3$

thus substitute:

$\displaystyle 189= \frac{ a_{1}(1 - {( \frac{1}{2} )}^{3} )}{1 - \frac{1}{2} }$

to figure out $a_1$ we need to figure out the equation

simplify denominator:

$\displaystyle \frac{ a_{1}(1 - {( \frac{1}{2} )}^{3} )}{ \dfrac{1}{2} } = 189$

simplify square:

$\displaystyle \frac{ a_{1}(1 - {( \frac{1}{8} )}^{} )}{ \dfrac{1}{2} } = 189$

simplify substraction:

$\displaystyle \frac{ a_{1} (\frac{7}{8} )}{ \frac{1}{2} } = 189$

simplify complex fraction:

$\displaystyle a_{1} (\frac{7}{8} ) \div { \frac{1}{2} } = 189$

calculate reciprocal:

$\displaystyle a_{1} \frac{7}{8} \times 2 = 189$

reduce fraction:

$\displaystyle a_{1} \frac{7}{4} \ = 189$

multiply both sides by 4/7:

$\displaystyle a_{1} \frac{7}{4} \times \frac{4}{7} \ = 189 \times \frac{4}{7}$

reduce fraction:

$\displaystyle a_{1} = 27\times 4$

simplify multiplication:

$\displaystyle a_{1} = 108$

hence,

$\displaystyle a_{1} = 108$