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问题: 叠放

解答:

(1) ∵9/6 = 3/2,即相邻三角形遮住对方底边的2/3
  --->相邻三角形的间隔 = BB'=(1/3)BC = 2
(2) Δ = 6*9/2 = 27
  S2 = Δ+[Δ-(2/3)^Δ] = 27+15 = 42
(3) S3 = S2+(1-4/9)Δ = 42+15 = 57
  S4 = S3+15 = 72
(4) Sn = 27+15(n-1) = 12+15n