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U ₇ = a + 6b
U₁₀ = a + 9b
------------------- +
48 = 2a + 15b .......(2)
eliminasikan pers (1) dan (2)
a + 4b = 10 x2 ===> 2a + 8b = 20
2a + 15b = 48 x1 ===> 2a + 15b = 48
-------------------- -
-7b = -28
b = 4
pers (1)
a + 4b = 10 ===> a + 4 (4) = 10
a + 16 = 10
a = -6
U₉ = a + 8b
= -6 + 8 (4)
= -6 + 32
= 26
Sn = n/2 x (a + Un)
S₉ = 9/2 x (-6 + 26)
= 9/2 x 20
= 90
semoga jelas dan membantu ya
u7+u10 =48
a+(7-1)b+a+(10-1)b=48
a+6b+a+9b =48
2a+15b =48
-------------------------------------persamaan 2
u5 =10
a+(5-1)b =10
a+4b =10
--------------------------------------eleminasi pers 1 dan 2
2a+15b =48
a+4b =10 ------>×2
---------------------------------------
2a+15b =48
2a+8b =20
--------------------------------------- kurangi
7b=28
b =28:7
b =4
--------------------------------------mencari awal(a)
u5 =10
a+(n-1)b =10
a+(5-1)4 =10
a+16 =10
a =10-16
a =-6
-------------------------------------membuat barisan
-6,-2,2,6,10,14,18,22,26,30
-------------------------------------mencari S9
S9=n/2(a+un)
S9=9/2(-6+26)
S9=9/2 × 20
S9=90
Kalo bisa jadiin jawaban terbaik ya.... terimakasih