Diketahui barisan aritmatika, jika U3= 42, U5+U9=52 maka S20=...?
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Pertanyaan
Diketahui barisan aritmatika, jika U3= 42, U5+U9=52 maka S20=...?
1 Jawaban
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1. Jawaban nisa2421
Un=a+(n-1)b
U3=a+2b=42
U5+U9=a+4b+a+8b=2a+12b=52
a+2b=42 |×2| 2a+4b=84
2a+12b=52|×1|2a+12b=52
------------------------------------- -
-8b=32
b=32/-8
b=-4
a+2b=42
a+2(-4)=42
a-8=42
a=42+8=50
Sn=2a+(n-1)b
S20=2(50)+(20-1)-4
=2(50)+19(-4)
=100-76
=24