Oblicz pierwszy wyraz ciągu geometrycznego (an), jeśli:
a) q=2, S4=60
b)q=0.5, S4=2046
c) q=1, S4=460
a)
q = 2, S4 = 60
n = 4
a1 = ?
Sn = a1 * (1-q^n)/(1-q)
60 = a1 *(1-2^4)/(1-2)
a1 *(1-16)/(-1) = 60
15a1 = 60 /:15
a1 = 4
=====
b)
q = 0,5, S4 = 2046
Sn = a1 *(1-q^n)/(1-q)
2046 = a1 *(1-0,5^4)/(1-0,5)
a1*(1-0,0625)/0,5 = 2046
a1 * 9.9375/0,5 = 2046
1,875a1 = 2046 /:1,875
a1 = 1091,2
========
c)
q = 1 , S4 = 460
dla q = 1
Sn = n*a1
460 = 4a1 /:4
a1 = 115
======
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a)
q = 2, S4 = 60
n = 4
a1 = ?
Sn = a1 * (1-q^n)/(1-q)
60 = a1 *(1-2^4)/(1-2)
a1 *(1-16)/(-1) = 60
15a1 = 60 /:15
a1 = 4
=====
b)
q = 0,5, S4 = 2046
n = 4
a1 = ?
Sn = a1 *(1-q^n)/(1-q)
2046 = a1 *(1-0,5^4)/(1-0,5)
a1*(1-0,0625)/0,5 = 2046
a1 * 9.9375/0,5 = 2046
1,875a1 = 2046 /:1,875
a1 = 1091,2
========
c)
q = 1 , S4 = 460
n = 4
a1 = ?
dla q = 1
Sn = n*a1
460 = 4a1 /:4
a1 = 115
======