evaluate 12∑n=3 -5n-1

help me step by step please

Question

evaluate 12∑n=3  -5n-1

help me step by step please

anonymous · Answer

did u try typing the exact same thing on google...usually answer.com give you good answers :)

anonymous · Answer

hold on..let me see though

anonymous · Answer

actually  have, but nothing came up for me :(

misty1212 · Answer

HI!!

misty1212 · Answer

something is not clear in the question

misty1212 · Answer

$$12\sum_{n=3}^?(-5n-1)$$

anonymous · Answer

the 12 is where the question mark is

misty1212 · Answer

ooh

misty1212 · Answer

$$\sum_{n=3}^{12}(-5n-1)$$

anonymous · Answer

exactly that lol. can you explain it to me?

misty1212 · Answer

you have to add up a bunch of stuff

anonymous · Answer

guide me through it step by step?

misty1212 · Answer

we can break it apart in to a couple pieces

anonymous · Answer

alright :)

misty1212 · Answer

$$-5\sum_{n=3}^{12}n-\sum_{n=3}^{12}1$$

anonymous · Answer

im confused, what method is this? this wasnt in the lesson :o

misty1212 · Answer

ok then lets do it a different way

misty1212 · Answer

the first number is what you get when $n=3$

misty1212 · Answer

$$-5\times 3-1=-16$$

misty1212 · Answer

the last number is what you get when $n=12$
$$-3\times 12-1=-37$$

misty1212 · Answer

there are 10 numbers between 3 and 12 inclusive

misty1212 · Answer

so you can use 
$$\frac{10}{2}\left(-16-37)\right)$$

misty1212 · Answer

i messed up womewhere hold on

anonymous · Answer

if  i use that formula, will it give me the total of the sum?

misty1212 · Answer

the last number is what you get when $n=12$ should be 
$$-5\times 12-1=-61$$

misty1212 · Answer

that looks better

misty1212 · Answer

$$\frac{10}{2}\left(-16-61\right)$$

misty1212 · Answer

http://www.wolframalpha.com/input/?i=5%28-16-61%29

misty1212 · Answer

here it is without the formula, same answer from wolfram http://www.wolframalpha.com/input/?i=sum+n+%3D+3+to+12%28-5n-1%29

anonymous · Answer

ohhhhhh, thanks a lot thats the answer i got. can you help me another one?

misty1212 · Answer

k

anonymous · Answer

5∑ n=1 3(-2)^n-1


i got  33 but i wanna check and make sure if i did this correctly

misty1212 · Answer

is the five on top?

anonymous · Answer

yes

misty1212 · Answer

33 is right

anonymous · Answer

how? can i see the steps?

misty1212 · Answer

however you did it

misty1212 · Answer

5 is a small enough number you can just add

anonymous · Answer

alright thanks, can you help with one last question?

misty1212 · Answer

$$3\times 1+3\times (-2)+3\times 4+3\times (-8)+3\times 16$$

misty1212 · Answer

ok but this one will cost you

misty1212 · Answer

jk

anonymous · Answer

LOL thanks a lot really!~

i have tons of doubts on this one really. 

given the sequence 5,-10,20...  determine the sigma notation  of the sum for term  4 through 15

anonymous · Answer

its asking for the formula on what would it be

anonymous · Answer

@misty1212

misty1212 · Answer

$$\sum_{n=4}^{15}5\times (2)^{n-1}$$ should do it

misty1212 · Answer

on no

misty1212 · Answer

$$\huge \sum_{n=4}^{15}5\times (2)^{n-4}$$

misty1212 · Answer

check to see if the first three terms are right

anonymous · Answer

that isnt among the options they give me for this question. the 2 in the parenthesis is negative and the exponent is n-1

anonymous · Answer

@misty1212

anonymous · Answer

@misty1212  are you still there?

anonymous · Answer

@mathrulezz

anonymous · Answer

im sorry! i dont' know how to do those :(