The 1st row has 15 chairs.
The 2nd row has 15 + 3 = 18 chairs.
The 3rd row has 18 + 3 = 15 + 2•3 = 21 chairs.
The 4th row has 21 + 3 = 15 + 3•3 = 24 chairs.
The pattern continues, so that the n-th row has 15 + (n - 1)•3 = 3n + 12 chairs.
If there are 15 rows, then the total number of chairs is
![\displaystyle \sum_{n=1}^{15} (3n + 12) = 3 \sum_{n=1}^{15} n + 12 \sum_{n=1}^{15} 1 = 3\cdot\frac{15\cdot16}2 + 12\cdot15 = \boxed{540}](https://tex.z-dn.net/?f=%5Cdisplaystyle%20%5Csum_%7Bn%3D1%7D%5E%7B15%7D%20%283n%20%2B%2012%29%20%3D%203%20%5Csum_%7Bn%3D1%7D%5E%7B15%7D%20n%20%2B%2012%20%5Csum_%7Bn%3D1%7D%5E%7B15%7D%201%20%3D%203%5Ccdot%5Cfrac%7B15%5Ccdot16%7D2%20%2B%2012%5Ccdot15%20%3D%20%5Cboxed%7B540%7D)
where we use the formulas
![\displaystyle \sum_{n=1}^N 1 = N](https://tex.z-dn.net/?f=%5Cdisplaystyle%20%5Csum_%7Bn%3D1%7D%5EN%201%20%3D%20N)
![\displaystyle \sum_{n=1}^N n = \frac{N(N+1)}2](https://tex.z-dn.net/?f=%5Cdisplaystyle%20%5Csum_%7Bn%3D1%7D%5EN%20n%20%3D%20%5Cfrac%7BN%28N%2B1%29%7D2)
Answer:
flute . It has some holes to blow
Answer: evaluate
Explanation:
Trailing the Text is a technique useful to analyze how a notable event is depicted in a text, meaning how it´s introduced, elaborated, and illustrated.
The introduction refers to the presentation of the event. The elaboration provides further details about it, and the illustration refers to the pictures that represent it.
Answer:
The Sun's gravity pulls the planets in orbit around it, and some planets pull moons in orbit around them. Even spacecraft are in motion through the solar system, either in orbit around the Earth or Moon, or traveling to further worlds, because of gravitational forces.
Explanation:
hope it helps you and give me a brainliest
Answer:
I would help put the picture is to blurry maybe try to take the picture a little bit close