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O是半圆的圆心.
勾股定理:t² = OB² - (x/2)²
t = √[16 - x²/4] = (1/2)√[64 - x²] (x≤8)
三角形面积关系:(1/2)×(x/2)×t = (1/2)×(OB/2)×h
h = (x/8)×√[64 - x²]
勾股定理:u² = x² - h²
u = x²/8
梯形面积关系:2×(1/2)OBh + (1/2)vh = (1/2)×2(u + v)h (等号右边是梯形面积公式)
v = 8 - 2u = 8 - x²/4 (x≤4√2)
y = 2x + 8 + v
= 2x + 8 + 8 - x²/4
= -(1/4)x² + 2x + 16 (x≤4√2)
当x = 4√2时,v = 0,梯形蜕化成三角形.
当4√2<x<8时,AD、BC交叉.
当x = 8时,腰AD或BC与AB重合.
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