Find S20 of the series whose first terms are: 30, 22, 14, 6, ...
1 answer:
Answer:
- 920
Step-by-step explanation:
There is a common difference d between consecutive terms in the sequence, that is
d = 22- 30 = 14 - 22 = 6 - 14 = - 8
This indicates the sequence is arithmetic with sum to n terms given by
=
[ 2a₁ + (n - 1)d ]
where a₁ is the first term and d the common difference
Here a₁ = 30 and d = - 8 , thus
=
[ (2 × 30) + (19 × - 8) ]
= 10(60 - 152)
= 10 × - 92
= - 920
You might be interested in
check the picture below, that's pretty much the same block, or "rectangular prism", with a width of 10, length of 5 and height of 1.
so anyhow, is really 6 rectangles stacked up to each other at the edges, so, we can simply get the area of each of those rectangles, add them up and that's the surface area.
front and back, two rectangles of 5x1
left and right, two rectangles of 1x10
top and bottom, two rectangles of 10x5
f(2) = 18