Question

Sa se determine x din 1+5+9+..+x=231

Answer 1

S=1+5+9+....x S=x+(x-4) +.....1 adunam sumele => 2S=(x+1)+(x+1)+.....(x+1) . SUma are (x-1)/4+1 =(x-1+4)/4=(x+3)/4 termeni => S=(x+1)(x+3)/4*1/2=231 => x²+3x+x+3=231*8 => x²+4x-1845=0; Δ=16+7380=7396. √Δ =86 => x=(-4+86)2=41 =>x=41. sa facem proba S=1+5+.....41 =42*11/2=462/2=231