Answer:
[tex] \displaystyle a_{1} = 108[/tex]
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
[tex] \displaystyle S_{ \text{n}} = \frac{ a_{1}(1 - {r}^{n} )}{1 - r} [/tex]
we are given that
- [tex]S_n=189[/tex]
- [tex]r=\dfrac{1}{2}[/tex]
- [tex]n=3[/tex]
thus substitute:
[tex] \displaystyle 189= \frac{ a_{1}(1 - {( \frac{1}{2} )}^{3} )}{1 - \frac{1}{2} } [/tex]
to figure out [tex]a_1[/tex] we need to figure out the equation
simplify denominator:
[tex] \displaystyle \frac{ a_{1}(1 - {( \frac{1}{2} )}^{3} )}{ \dfrac{1}{2} } = 189[/tex]
simplify square:
[tex] \displaystyle \frac{ a_{1}(1 - {( \frac{1}{8} )}^{} )}{ \dfrac{1}{2} } = 189[/tex]
simplify substraction:
[tex] \displaystyle \frac{ a_{1} (\frac{7}{8} )}{ \frac{1}{2} } = 189[/tex]
simplify complex fraction:
[tex] \displaystyle a_{1} (\frac{7}{8} ) \div { \frac{1}{2} } = 189[/tex]
calculate reciprocal:
[tex] \displaystyle a_{1} \frac{7}{8} \times 2 = 189[/tex]
reduce fraction:
[tex] \displaystyle a_{1} \frac{7}{4} \ = 189[/tex]
multiply both sides by 4/7:
[tex] \displaystyle a_{1} \frac{7}{4} \times \frac{4}{7} \ = 189 \times \frac{4}{7} [/tex]
reduce fraction:
[tex] \displaystyle a_{1} = 27\times 4[/tex]
simplify multiplication:
[tex] \displaystyle a_{1} = 108[/tex]
hence,
[tex] \displaystyle a_{1} = 108[/tex]
