Răspuns:
Explicație pas cu pas:
progresie aritmetica
suma=155
r=3
an=x
an=a1+r(n-1)⇒x=2+3(n-1)⇒3n=x-2+3⇒n=(x+1)/3
s=(a1+an)*n/2
155=[(2+x)*(x+1)/3]/2
155=(2x+2+x²+x)/6
930=x²+3x+2
x²+3x-928=0
Δ=9+4*928=9+3712=3721
x₁=(-3+√3721)/2=(-3+61)/2=58/2=29
x₂=(-3-√3721)/2=(-3-61)/2=-64/2=-64/2=-32
verificare:
s₁=(2+29)*10/2=31*5=155
s₂=(2-32)*(-31/3)/2 ≠155
