Triangular ADC is similar to triangular HGC,

So AD/DC=HG/GC. That is, a/a+b = Hg/B.

So HG=ab/(a+b)

，BH=b-HG=b-ab/(a+b)

Take BH as the base of triangle abc, and s = {b-ab/(a+b)} times (a+b) and divides by 2.

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