Find the sum of the first 15 terms in the sequence using the appropriate formula. Show the formula and the steps used to get your answer

Question

Find the sum of the first 10 terms in the sequence using the appropriate formula. Show the formula and the steps used to get your answer

-3, -4.5, -6, -7.5…

d= -1.5

Answer 1

The "formulas:

Use the top formula to get the sum of the first 15 terms (meaning that n = 15). You will notice that in order to use the formula, you must know the 15th term (a₁₅). To determine what a₁₅ is, you can use the second formula.

a₁₅ = a₁ +(15 - 1)(d)

plugging in a₁ = -3 and d = -1.5 we get:

a₁₅ = -3 +(15 - 1)(-1.5)

Once you get that value, plug it into the top equation:

s₁₅ = 15(-3 + a₁₅)/2