SOLUTION: Hello, I don't know if this is a linear equation or not, sorry. My question is: I need a formula to calculate the total cost of something. But the more I have, the more it costs

Question 836583: Hello, I don't know if this is a linear equation or not, sorry.
My question is: I need a formula to calculate the total cost of something. But the more I have, the more it costs.
Example:
Price per unit: $10
Price for second unit: $15
Price for third unit: $20
So for every extra unit I have, it will add $5.
Formula/equation I need is:
Find Total cost. I have 5 units on hand ($100). I need to purchase another 10 units. So it should add $105 (6th unit) + $110 + $115 ... + $145 (9th unit) + $150 (10th unit).
Would be nice to have a formula where I can plug in the numbers. For example, I have 5 units on hand so my next starting cost would be $105, and I will buy 7 more units, then my total price would be x.
Thank you very much for your time.
Answer by richard1234(7193) (Show Source): You can put this solution on YOUR website!
The n-th unit costs $5n+5. Also, the cost for 5 units is 10+15+...+30 = $100, but be careful; the sixth unit costs $35, not $105.

For n >= 1, the cost for n units is

This is one of those "Gauss sums," and the sum is

.
RELATED QUESTIONS
Need help with the basics of logarithm. 5log 7a + 20b I don't know how to format... (answered by ikleyn)

Hello, My question is about congruent triangles. I don't know if it is possible to... (answered by ankor@dixie-net.com,math_helper)

Hello there, I need help with this math question. I know how to do the question but I... (answered by josgarithmetic)

I think this is a basic question. But it has been 10 years since I had any algebra and I... (answered by josmiceli,ikleyn)

I am investigating a pattern, our topic is linear patterns. The pattern is a square like... (answered by josgarithmetic)

hi! i really need help with this question... it's not a number problem question...it's a... (answered by rapaljer)

A builder of tract homes reduced the price of a model by 15%. If the new price is... (answered by ikleyn,MathTherapy,greenestamps)

Sorry guys I did not finish my last post out. I am new here. This is a repost with the... (answered by josmiceli)

Hello again my friends. Can someone help me solve this problem ? Also I would like to... (answered by josgarithmetic)