2·(c + d) = a·b --> d = a·b/2 - c
2·(a + b) = c·d
2·(a + b) = c·(a·b/2 - c)
c2 - 1/2·a·b·c + 2·(a + b) = 0
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c2 - 1/2·a·b·c + 2·(a + b) = 0
c2 - 1/2·a·b·c = - 2·(a + b)
c2 - 1/2·a·b·c + (1/4·a·b)2 = (1/4·a·b)2 - 2·(a + b)
(c - 1/4·a·b)2 = 1/16·a2·b2 - 2·(a + b)