Odpowiedzi

2010-03-05T17:00:03+01:00
Sn=204
r=6
An=49

Sn= [(a1+an)/2]*n
an= a1+(n-1)*r

Sn= [(a1+a1+(n-1)*r)/2]*n

204= [(2a1+6n-6)/2]*n i
49= a1+6n-6
55= a1+6n
-a1=-55+6n
a1= 55-6n

204= [(2*(55-6n)+6n-6)/2]*n
204= [(110-12n+6n-6)/2]*n/*2
408= (104-6n)*n
408= 104n-6n²

6n²-104n+408=0
Δ= 10816- 9792= 1024
√Δ= 32
n1= 104-32/12= 72/12= 6
n2= 104+32/12= 136/12= 11,3

Odp. Ten ciąg ma sześć wyrazów .
1 5 1