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八年级数学解答题一般
题目
如图,ADADABC\triangle ABC的角平分线,过点DD分别作ACACABAB的平行线,交ABAB于点EE,交ACAC于点FF.
(1)(1)求证:四边形AEDFAEDF是菱形.
(2)(2)AF=13AF=13,AD=24AD=24.求四边形AEDFAEDF的面积.
知识点:三角形的面积、菱形的判定与性质章节:第21章 四边形 / 21.3 特殊的平行四边形 / 21.3.2 菱形

答案与解析

答案

(1)(1)证明:AB\because ABDF,ACDF,ACDEDE
\therefore四边形AEDFAEDF是平行四边形.
AD\because ADABC\triangle ABC的角平分线,
BAD=DAC\therefore \angle BAD=\angle DAC.
AC\because ACDEDE
ADE=DAC\therefore \angle ADE=\angle DAC.
ADE=BAD\therefore \angle ADE=\angle BAD.
EA=ED\therefore EA=ED.
\therefore四边形AEDFAEDF是菱形.
(2)(2)连接EFEFADAD于点OO.

\because四边形AEDFAEDF是菱形,
EF=2FO\therefore EF=2FO.
AO=12AD=12\therefore AO=\frac{1}{2}AD=12.
ADEF\because AD\bot EF.
RtAOFRt\triangle AOF中,由勾股定理得OF=AF2AO2=132122=5OF=\sqrt{A{F}^{2}-A{O}^{2}}=\sqrt{1{3}^{2}-1{2}^{2}}=5.
OE=OF=5\therefore OE=OF=5.
\therefore四边形AEDFAEDF的面积=12AD×OF+12AD×OE=12×24×5+12×24×5=120=\frac{1}{2}AD×OF+\frac{1}{2}AD×OE=\frac{1}{2}×24×5+\frac{1}{2}×24×5=120.

解析

(1)(1)证明:AB\because ABDF,ACDF,ACDEDE
\therefore四边形AEDFAEDF是平行四边形.
AD\because ADABC\triangle ABC的角平分线,
BAD=DAC\therefore \angle BAD=\angle DAC.
AC\because ACDEDE
ADE=DAC\therefore \angle ADE=\angle DAC.
ADE=BAD\therefore \angle ADE=\angle BAD.
EA=ED\therefore EA=ED.
\therefore四边形AEDFAEDF是菱形.
(2)(2)连接EFEFADAD于点OO.

\because四边形AEDFAEDF是菱形,
EF=2FO\therefore EF=2FO.
AO=12AD=12\therefore AO=\frac{1}{2}AD=12.
ADEF\because AD\bot EF.
RtAOFRt\triangle AOF中,由勾股定理得OF=AF2AO2=132122=5OF=\sqrt{A{F}^{2}-A{O}^{2}}=\sqrt{1{3}^{2}-1{2}^{2}}=5.
OE=OF=5\therefore OE=OF=5.
\therefore四边形AEDFAEDF的面积=12AD×OF+12AD×OE=12×24×5+12×24×5=120=\frac{1}{2}AD×OF+\frac{1}{2}AD×OE=\frac{1}{2}×24×5+\frac{1}{2}×24×5=120.

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