Answer: 
<u>Step-by-step explanation:</u>
a₁, 375, a₃, a₄, 81
First, let's find the ratio (r). There are three multiple from 375 to 81.
![375r^3=81\\\\r^3=\dfrac{81}{375}\\\\\\r^3=\dfrac{27}{125}\qquad \leftarrow simplied\\\\\\\sqrt[3]{r^3} =\sqrt[3]{\dfrac{27}{125}}\\ \\\\r=\dfrac{3}{5}](https://tex.z-dn.net/?f=375r%5E3%3D81%5C%5C%5C%5Cr%5E3%3D%5Cdfrac%7B81%7D%7B375%7D%5C%5C%5C%5C%5C%5Cr%5E3%3D%5Cdfrac%7B27%7D%7B125%7D%5Cqquad%20%5Cleftarrow%20simplied%5C%5C%5C%5C%5C%5C%5Csqrt%5B3%5D%7Br%5E3%7D%20%3D%5Csqrt%5B3%5D%7B%5Cdfrac%7B27%7D%7B125%7D%7D%5C%5C%20%5C%5C%5C%5Cr%3D%5Cdfrac%7B3%7D%7B5%7D)
Next, let's find a₁
Lastly, Use the Infinite Geometric Sum Formula to find the sum:
The slope is -1/2x in this case because it is always next to a variable y=mx+b.
The appropriate formula here is l = a + (n-1)d, where l represents the last term.
Here the first term is -5 and the common difference is +4. Thus,
l = -5 + (n-1)(4) (answer)
Check: we are given the arith sequence -5, -1, 3, 7. Does our formula correctly predict the 1st term? l = -5 + (1-1)(4) = > -5. Yes.
Does it correctly predict the 3rd term? l = -5 +(3-1)(4) => -5+2(4) = 3. Yes
