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Question

thatonegirl_ · Answer

Find value of n
$$\sum_{i=1}^{n}(-5+7i)=486$$

fibonaccichick666 · Answer

So, what is the first value of the sequence(n=1?)?

thatonegirl_ · Answer

2

thatonegirl_ · Answer

@FibonacciChick666

fibonaccichick666 · Answer

yea, that's right, so how about when n=2?

thatonegirl_ · Answer

9 @FibonacciChick666

fibonaccichick666 · Answer

good, so now if you add those together, what do you get?

thatonegirl_ · Answer

11

fibonaccichick666 · Answer

right, so now we just think for a moment,  if we are successively adding them together, what is your guess for what we need -5+7i to equal when the summation will equal 486?

thatonegirl_ · Answer

I have no idea

fibonaccichick666 · Answer

So, we just want to try guessing to get close

thatonegirl_ · Answer

12 @FibonacciChick666

fibonaccichick666 · Answer

ok so let's see if it seems reasonable,  if n=12 we get -5+7(12)=which is two more than 7(11) right?

fibonaccichick666 · Answer

so that's 79, does it seem plausible that the numbers less than 79 in our sequence add up to 486?

anonymous · Answer

use this link it will give you your answer. http://tutorial.math.lamar.edu/Classes/CalcI/SummationNotation.aspx