tolong dong yang nomor17

S9 = [tex] \frac{9}{2} [/tex] (2a + 8b) = 45
=> 9a + 36b = 45
=> a + 4b = 5

S5 = [tex] \frac{5}{2} [/tex] (2a + 4b) = 20
=> 5a + 10b = 20
=> a + 2b = 4

a + 4b = 5
a + 2b = 4
--------------- -
2b = 1 => b = [tex] \frac{1}{2} [/tex]

a + 2b = 4
=> a + 2([tex] \frac{1}{2} [/tex]) = 4
=> a = 3

jadi suku pertama = 3