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Kelas IX
a = 205
Un = 395
b = 5
Un = a+(n-1)b
395 = 205 + 5n - 5
5n = 195
n = 39
Sn = n/2 (a+Un)
S39= 39/2 (205+395)
S39= 39/2 x 600
S39 = 11.700
Kelas : IX SMP
Bab : Barisan dan Deret
Pembahasan :
205 , 210 , 215 , ... 395
Un = 395
a + (n - 1)b = 395
205 + 5n - 5 = 395
5n + 200 = 395
5n = 195
n = 39
Maka...
Sn = n/2(2a + (n - 1)b)
S39 = 39/2(2.205 + 38.5)
S39 = 39/2(410 + 190)
S39 = 39/2(600)
S39 = 11.700