Evaluate enter only the sum

Question

S₂₁for 23+20+17+14+.....

Answer 1

S_n = (n/2)[2a₁ + (n - 1)d], where n = number of terms, a₁ = first term, d = common difference

 

S₂₁ = (21/2)[46 + 20(-3)]

S₂₁ = (21/2)(46 - 60)

S₂₁ = (21/2)(-14)

S₂₁ = -147