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The answer of the seventh grade math master
(1) Because the triangle ABD is composed of AB, AD and BD.

Triangle ACD consists of AC, AD and CD.

Because AD is the center line of triangle ABC.

So CD = BD

So the difference between the perimeters of triangle ABD and triangle ACD is 5-3 = 2 cm.

(2) Because the area of triangle ABD is 1/2*AE*BC.

The area of triangle ACD is 1/2*AE*CD.

And because BC=2CD.

So the area of triangle ABD is twice that of triangle ACD.