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Mashcka [7]
3 years ago
8

-2,-7,-12 ,.... is an arithmetic sequence Find the sum of its first 20 terms ?​

Mathematics
1 answer:
nlexa [21]3 years ago
4 0

Given sequence:

-2,-7,-12 ,..

Unknown;

Find the sum of the first 20 terms = ?

Solution:

  The sequence is -2,-7,-12 ,..

To find he sum of n-terms of a sequence, we use the expression below;

     Sₙ  = \frac{n}{2} (2a + (n-1)d)

where n is the number of terms = 20

           a is the first term = -2

            d is the common difference= -7 - (-)2 = -5

Now input the parameters and solve;

       Sₙ  = \frac{20}{2} (2(-2) + (20 - 1) x -5)

      Sₙ = 10(-4 + -5(19))  = 10(-99)  = -990

The sum of the first 20 terms of the sequence is -990

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Delicious77 [7]

Answer:

The answer is B

Step-by-step explanation:

i took the test and got it right on edge Give me brainiest please :)

5 0
2 years ago
You go shopping at a local department store with your friend and cousin. You buy one pair of jeans, four pairs of shorts, and tw
VashaNatasha [74]

Answer: Cost per pair of jeans = $ 20

Cost per pair of shorts = $12

Cost per shirt = $8

Step-by-step explanation:

Let x= cost per pair of jeans

y = Cost per pair of shorts

z= cost per shirt.

For you, total cost = x+4y+2z= 84

For your friend,  total cost = 2x+y+3z=76

For cousin, total cost = x+2y+z= 52

Required system:

x+4y+2z=84\ (i)\\ 2x+y+3z=76\ (ii)\\ x+2y+z=52\ (iii)

from (i), x=84-4y-2z    (*)

put this in (ii) and (iii) we get

2\left(84-4y-2z\right)+y+3z=76\\\Rightarrow\ -7y-z+168=76\ (iv)\\\\ 84-4y-2z+2y+z=52\\\Rightarrow\ -2y-z+84=52 \ (v)

from (iv), y=-\frac{z-92}{7}\ (vi)

\mathrm{Subsititute\:}y=-\frac{z-92}{7}\ \text{in}\ (v)

-2\left(-\frac{z-92}{7}\right)-z+84=52\\\\\Rightarrow\ -2\left(-\frac{z-92}{7}\right)-z=-32\\\\\Rightarrow\ -\frac{5z}{7}-\frac{184}{7}=-32\\\\\Rightarrow\ -5z-184=-224\\\\\Rightarrow\ -5z=-40\\\\\Rightarrow\ z=8

\mathrm{Subsititute\:}z=8\text{ in (vi)}

y=-\frac{8-92}{7}\\\\\Rightarrow \ y=12

\mathrm{Subsititute\:}z=8,\:y=12\text{ in (*)}

x=84-4\cdot \:12-2\cdot \:8\\\\\Rightarrow\ x= 20

Hence, x= 20, y= 12 , z=8

So, Cost per pair of jeans = $ 20

Cost per pair of shorts = $12

Cost per shirt = $8

7 0
3 years ago
Where do u put parentheses yo make the equation 10+9×3-1×2 true?
myrzilka [38]

Answer

((10+9)x3-1)x2

=(19x3-1)x2

=(57-1)x2

=56x2=112

5 0
3 years ago
31
VashaNatasha [74]
The answer is , c. 3.4 lol
6 0
3 years ago
1/2(8-x)=10<br> A.12<br> B.3<br> C.-12
Murljashka [212]
4-1/2x=10
-1/2x=6
3
so your answer would be B
hope that helps
7 0
3 years ago
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