Find a1 if s20=450 d=5

Answer 1

By S₂₀, I assume that you mean the sum of the first 20 terms of the sequence.

In that case, S₂₀ = (20/2)[a₁ + a₂₀] = 450

So, 10[a₁ + a₁ + 19d] = 450

2a₁ + 19(5) = 45

2a₁ = -50

a₁ = -25

Answer 2

s20=a1 +(20-1)*d

450 = a1 +(20-1)*5

a1= 355