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Un = 37
U10 - U5 = 15
(a+9b) - (a+4b) = 15
a + 9b - a - 4b = 15
5b = 15
b 3
A.
Un = a+(n-1)b
37 = -5 + (n-1)3
37 = -5 + 3n - 3
37 = 3n - 8
3n = 8 + 37
3n = 45
n = 15
B.
S15 = 15/2 (2(-5) + (14)3)
= 15/2 (-10 + 42)
= 15/2 ( 32)
= 15(16)
= 240
maaf kalau salah. periksa ulang lagi ya