Area of a quadrilateral

Geometry Level 3

In quadrilateral A B C D ABCD shown above, point E E is the midpoint of A C AC . Given that the area of A B C = 1 \triangle ABC=1 and the area of B C D = 4 \triangle BCD=4 , find the area of quadrilateral A B C D ABCD .

Note: B E A D BE \parallel AD


The answer is 7.

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1 solution

Extend B E BE to C D CD at point G G . Since point E E is the midpoint of A C AC , area B C E = 0.5 \triangle BCE =0.5 and area B A E = 0.5 \triangle BAE =0.5 . Since G G is the midpoint of C D CD , area B C G = 2 \triangle BCG=2 and area B G D = 2 \triangle BGD=2 .

area C E G = 2 0.5 = 1.5 \triangle CEG=2-0.5=1.5

Since C E G C A D \triangle CEG \sim \triangle CAD , area C A D = 4 × \triangle CAD =4\times area C E G \triangle CEG , so

area C A D = 4 ( 1.5 ) = 6 \triangle CAD=4(1.5)=6

Finally,

area A B C D = A B C + C A D = 1 + 6 = ABCD=\triangle ABC + \triangle CAD=1+6= 7 \color{#D61F06}\boxed{\large 7}

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