3-6 The internal bisector theorem and its converse
Point 6 of 9 — Unit 3: Parallel Lines, Proportional Segments and Angle BisectorsQuestion bank: 0 questions (0 with pictures)
06 3-6 The internal bisector theorem and its converse
Theorem: if AD bisects angle BAC of triangle ABC and D lies on BC, then BD/DC = AB/AC — the bisector divides the opposite side in the ratio of the two sides forming the angle.
Mnemonic: the ratio in which the bisector cuts the opposite side equals the ratio of the adjacent sides.
Converse: if D on BC satisfies BD/DC = AB/AC, then AD bisects angle BAC.
Rearrangements used in exams: BD = BC·AB/(AB + AC), DC = BC·AC/(AB + AC), and BD/BC = AB/(AB + AC).
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