.
It is about finding the number of terms of an arithmetic progression, when its
first term, the common difference and the sum of n terms are given.
The formula for the sum of n first terms of an AP is
= ,
In this problem, = 10, d = 6. So your equation is
= 3920.
Multiply both sides by 2
(20 + 6*(n-1))*n = 7840
20n + 6n^2 - 6n = 7840
6n^2 + 14n - 7840 = 0.
Apply the quadratic formula and find the roots.
You will get two roots. One root is negative, and we reject it.
The other root is positive, and its value is 35.
It is the solution to the problem.
ANSWER. 35 rows.
Solved.
-----------------
On geometric progressions, see introductory lessons
- Geometric progressions
- The proofs of the formulas for geometric progressions
- Problems on geometric progressions
- Word problems on geometric progressions
in this site.
Also, you have this free of charge online textbook in ALGEBRA-II in this site
- ALGEBRA-II - YOUR ONLINE TEXTBOOK.
The referred lessons are the part of this online textbook under the topic
"Geometric progressions".
Save the link to this textbook together with its description
Free of charge online textbook in ALGEBRA-II
https://www.algebra.com/algebra/homework/complex/ALGEBRA-II-YOUR-ONLINE-TEXTBOOK.lesson
into your archive and use when it is needed.