Q:

Does the Slavic word rā́dъ have cognates in Indo-iranian?

I found a source which gives the PIE origin: http://www.utexas.edu/cola/centers/lrc/ielex/X/P1589.html
But it only lists Slavic reflexes. Are there related words in Persian or Sanskrit? 
Wikipedia claims the name of the Indian goddess Radha is related https://en.wikipedia.org/wiki/Radha#Etymology, but gives no source for this claim. 

A:

As so often, Wikipedia is completely wrong about this. The Sanskrit name Rādhā cannot possibly be related to Persian rād “generous”. Indo-Iranian post-vocalic dh becomes y in Persian, not d.

