Odpowiedzi

2010-04-12T22:48:19+02:00
2010-04-12T22:54:48+02:00
Sn= a1+a2...+an
Sn+4= a1+a2+..+an+3+an+4
Sn+4 - Sn = an+4 + an+3 + an+2 +an+1
an+4=an+4*4=an+2
an+3=an+3/2
an+2=an+1
an+1=an+1/2
Sn+4-Sn=124-81=43
4*an+5=43
4*an=38
an=19/2
an= a1 + (n-1)1/2
19/2=a1+(n-1)1/2 /*2
20=2a1+n
a1= (20-n)/2
z podanych sum mamy (podstawiajac an=19/2):
Sn=(a1+an)/2 *n=84
(20-n/2+19/2)/2 *n=84 /*4
39n-n²=324
n²-39n+324=0
Δ=225
n1 = 39+15/2 n2 = 39-15/2
n1=27 n2=12

Odp n=27 lub =12, a an =19/2
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