Ok well, you already have that angle 1 is congruent to angle 3, which is congruent to angle 2. If two angles are congruent, the sides opposite them are of equal length. I don't know how to "officially" contruct a proof, but I know that that's the reason why they are equal. XD
The fact that sides BC and CD are of equal length because the angles opposite them (angleDAC & angleCAB) is just a mathematical theorem. If two triangles have congruent angles it is possible that they have congruent sides as well. Because the sides in question lie on the same line (one is not "further away" from their angle than the other) it can be assumed that the sides are of equal length.
*shrug* I took geometry years ago, so I don't remember specifics. Sorry. =/
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Oh oh! I see it! But do you know what the *reason* why the other sides are congruent to those two angles?
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The fact that sides BC and CD are of equal length because the angles opposite them (angleDAC & angleCAB) is just a mathematical theorem. If two triangles have congruent angles it is possible that they have congruent sides as well. Because the sides in question lie on the same line (one is not "further away" from their angle than the other) it can be assumed that the sides are of equal length.
*shrug* I took geometry years ago, so I don't remember specifics. Sorry. =/
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Im sorry I cant help you though :( <3 good luck!
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I'm sure you will understand it, once you do a couple :)
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