if a5= -16, and d= -5, what's the explicit formula?

Question

I put a5 but I meant a₅ lol I didn't know how to type it

Answer 1

a₅ = a₁ + 4d

-16 = a₁ + (4)(-5)

-16 = a₁ - 20

4 = a₁

a_n = 4 + (n - 1)(-5)

Answer 2

Hi Chloë E.,⁴

I suppose you have an arithmetic sequence? Just count backwards to a₀ I think you want? That's 5 terms back = 5*(-(-5)) = 25. So a₀ = -16+25 = 9. Let's check: a₁ = 9-5 = 4; a₂ = 4-5 = -1; a₃ = -1-5 = -6; a₄ = -6-5 = -11; a₅ = -11-5 = -16. Yup!

If your course likes to number its sequences from a₁ instead, you have that in the list also. Personally I always thought a₀ made sense, but that's the computer geek in me.

By the way Chloë, if you can't get a symbol, just state what it does, and put the "argument" in parentheses, like this: asub(0) . For exponents, the universal text symbol is "^", so 2⁴ = 2^(4), for example.

-- Cheers, --Mr. d.