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30 = a1 + 9r + a1 + 19r + a1 + 29r
30 = 3a1 + 57r /3
10 = a1 + 19r
a1 = 10-19r
a39 = a1 + 38r = 10-19r +38r = 10+19r
S39 = a1 + a39 /2 *39
S39 = 10-19r +10 +19r /2 *39
S39 = 20/2 * 39 = 390