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4 a + 3 d + 6 a + 5 d = 42 ... 10 a + 8 d = 42 ... 5 a + 4 d = 21
3 a + 2 d + 9 a + 8d = 52 ... 12 a + 10 d = 52 ... 6 a + 5 d = 26
30 a + 24 d = 126
30 a + 25 d = 130
subtracting equations ... d = 4
... so , a = 1