I want to know why in Suratul Qamar they used Walattaraknaahaa

 I want to know why in Suratul Qamar they used Walattaraknaahaa... without bringing sukuun on the daal... thank you
In the two words لقد تركناها we have a دال  saakinah as the last letter of one word word, followed by تاء as the first letter of the second word.
The دال and تاء  have the same articulation point, but differ in some characteristics, making their relationship classified as two alike, or متجانسين .   When two letters have this relationship, and the first is saakin and the last letter of a word, and the second voweled and the first letter of the next juxtaposed word, the rule is idghaam.  The first letter (in this case a daal) is inserted into the second letter, and the second letter is pronounced as an emphasized letter-meaning it has a shaddah and is pronounced as the same letter twice, once saakin and the second time with the vowel.  This is called idghaam mutajaasisaan sagheer.  http://abouttajweed.com/kb/entry/956/
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