Question

Please i need an answer urgently. Sum of 3 terms are 252, first term is 192. Find common ratio

Answer 1

Answer:

$r=\frac{1}{4}\\\\r=-\frac{5}{4}$

Step-by-step explanation:

Geometric Sequences

The geometric sequence is given as:

$a_1, a_1r, a_1r^2, a_1r^3,..., a_1r^{n-1}$

Where n is the number of the term, n≥1, and r is the common ratio.

The sum of n terms of the geometric sequence is given by:

$\displaystyle S_n=a_1\frac{r^n-1}{r-1}$

We are given: S3=252, a1=192, thus substuting:

$\displaystyle 252=192\frac{r^3-1}{r-1}$

Dividing by 12:

$\displaystyle 21=16\frac{r^3-1}{r-1}$

Recall that:

$r^3-1=(r-1)(r^2+r+1)$

Substituting:

$\displaystyle 21=16\frac{(r-1)(r^2+r+1)}{r-1}$

Simplifying:

$21=16(r^2+r+1)=16r^2+16r+16$

Rearranging:

$16r^2+16r+16-21=0$

16r^2+16r-5=0

Rewriting:

$16r^2-4r+20r-5=0$

Factoring:

$4r(4r-1)+5(4r-1)=0$

$(4r-1)(4r+5)=0$

Solving:

$r=\frac{1}{4}\\\\r=-\frac{5}{4}$

Both solutions are valid