Sum of an infinite geometric sequence with the common ratio r, and the first term a1 is

we notice
-2 times -1/4=1/2,
1/2 times -1/4=-1/8
so a1=-2
r=-1/4 or -0.25
so the sum is

=

=

=

=
the sum is -8/5 or -1.6
we are given
airthematic sequence
we can use nth term formula

n is number of terms
an is nth term
d is common difference
a1 is first term
We are given
a7=40
we can use it



a18 = 106


we can subtract both equations



now, we can find a1

we can plug d=6


12th term:

now, we can plug values



so, 12th term is 70...........Answer
Answer:
18
Step-by-step explanation:
Since you know the perimeters are equal, write your equation.
x+6 + x+6 + x+6 = x + x+6 + 2x-6
combine like terms
3x + 18 = 4x
isolate your x by subtracting 3x from each side
18 = x
check your answer:
3x + 18 = 72
4x = 72
Law of cosines
:
The law of cosines establishes:

general guidelines:
The law of cosines is used to find the missing parts of an oblique triangle (not rectangle) when either the two-sided measurements and the included angle measure are known (SAS) or the lengths of the three sides (SSS) are known.
Law of the sines:
In ΔABC is an oblique triangle with sides a, b, and c, then:

The law of the sines is the relation between the sides and angles of triangles not rectangles (obliques). It simply states that the ratio of the length of one side of a triangle to the sine of the angle opposite to that side is equal for all sides and angles in a given triangle.
General guidelines:
To use the law of the sines you need to know either two angles and one side of the triangle (AAS or ASA) or two sides and an opposite angle of one of them (SSA).
The ambiguous case
:
If two sides and an angle opposite one of them is given, three possibilities may occur.
(1) The triangle does not exist.
(2) Two different triangles exist.
(3) Exactly a triangle exists.
If we are given two sides and an included angle of a triangle or if we are given 3 sides of a triangle, we can not use the law of the sines because we can not establish any proportion where sufficient information is known. In these two cases we must use the law of cosines