As shown in the figure, in the square ABCD, the diagonal AC and BD intersect at point E, AF bisects ∠ BAC, intersects BD at point F, and proves: EF + 12ac = ab

As shown in the figure, in the square ABCD, the diagonal AC and BD intersect at point E, AF bisects ∠ BAC, intersects BD at point F, and proves: EF + 12ac = ab

It is proved that: as shown in the figure, FM ⊥ AB at point m, ≁ AC ⊥ BD at point E, AE = 12ac, ≁ abd = ≁ CBD = 45 °, AF bisection ≁ BAC, ≁ EF = MF. AF = AF, ≌ RT △ AMF ≌ RT △ AEF (HL), ≌ AE = am, ≁ MFB = ≁ ABF = 45 °, ≁ MF = MB, ≁ MB = EF, ≌ EF + 12ac = MB + AE = MB + am = ab