(20 points)

find the sum of an arithmetic series with the given number of terms a1, and an

27.) 10 terms, a1=4, a10=31

28.) 15 terms, a1=17, a15=129

? explain better

Step-by-step explanation:

check above link.

graphically?   well, you graph them both, the solutions is where they both intersect each other, since there are two solutions, meaning they will meet twice, or namely the linear will run into the quadratic curve twice, and where they meet, those x,y coordinates are the solutions to the system.