Odpowiedź:
n = 21
[tex]a_{21} = a_1 + 20 r = 135[/tex] ⇒ [tex]a_1 = 135 - 20 r[/tex]
[tex]S_{21} = 0,5*( a + 1 + a_{21})*21 = 1575\\0,5*(135 - 20 r + 135)*21 = 1575[/tex]
(135 - 10 r)*21 = 1575 / : 21
135 - 10 r = 75
10 r = 60
r = 6
=====
[tex]a_1 = 135 - 20*6 = 15[/tex]
======================
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Odpowiedź:
n = 21
[tex]a_{21} = a_1 + 20 r = 135[/tex] ⇒ [tex]a_1 = 135 - 20 r[/tex]
[tex]S_{21} = 0,5*( a + 1 + a_{21})*21 = 1575\\0,5*(135 - 20 r + 135)*21 = 1575[/tex]
(135 - 10 r)*21 = 1575 / : 21
135 - 10 r = 75
10 r = 60
r = 6
=====
[tex]a_1 = 135 - 20*6 = 15[/tex]
======================
Szczegółowe wyjaśnienie: