The sum of the first four terms of the sequence is 22.
In this question,
The formula of sum of linear sequence is

The sum of the first ten terms of a linear sequence is 145
⇒ 
⇒ 145 = 5 (2a+9d)
⇒ 
⇒ 29 = 2a + 9d ------- (1)
The sum of the next ten term is 445, so the sum of first twenty terms is
⇒ 145 + 445
⇒ 
⇒ 590 = 10 (2a + 19d)
⇒ 
⇒ 59 = 2a + 19d -------- (2)
Now subtract (2) from (1),
⇒ 30 = 10d
⇒ d = 
⇒ d = 3
Substitute d in (1), we get
⇒ 29 = 2a + 9(3)
⇒ 29 = 2a + 27
⇒ 29 - 27 = 2a
⇒ 2 = 2a
⇒ a = 
⇒ a = 1
Thus, sum of first four terms is
⇒ 
⇒ 
⇒ S₄ = 2(2+9)
⇒ S₄ = 2(11)
⇒ S₄ = 22.
Hence we can conclude that the sum of the first four terms of the sequence is 22.
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Make a drawing with a right triangle.
The opposite side, y, is the height of the dam less 1.65 m
The base or adjacent side is 90 m
The angle between the two sides is 90 m.
Then tan (26) = y / 90
y = 90 tan(26) = 90 (0.4877) = 43.90 m
The height of the dam is 43.90m + 1.65m = 45.55 m
Answer:
174.6 ft
Step-by-step explanation:
It can be helpful to draw a diagram of the triangle we're concerned with. (See attached.)
We know the angle at the end of the shadow inside the triangle is 52°-22° = 30°. We assume the tree is growing straight up out of the hillside, so its angle with the hill inside the triangle is 90°+22° = 112°. Then the remaining angle between the shadow and the tree at the top of the tree is ...
180° -30° -112° = 38°
Now, we have the angle opposite the tree, and the angle opposite the known side length of the triangle (215 feet along the hill, AC in the diagram). This is enough information to usefully use the Law of Sines.
c/sin(C) = a/sin(A)
c = a(sin(C)/sin(A)) = (215 ft)(sin(30°)/sin(38°)) ≈ 174.6 ft
The height of the tree is about 174.6 feet.