1) a3+a7+a18 = 48
а1+2d+a1+6d+a1+17d = 48
3×a1 + 25d = 48
2) S 20 = (3×a1 +25d)/2×20 = 24×10 = 240.
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1) a3+a7+a18 = 48
а1+2d+a1+6d+a1+17d = 48
3×a1 + 25d = 48
2) S 20 = (3×a1 +25d)/2×20 = 24×10 = 240.