The Tantrasamgraha (chapter 2 verse 12½) of Nilakantha and Karanapaddhati ( chapter 6 verse 19) have both given that for small arc jivā = s-s³/ 3! r² ( approximately )
Since sine values are not actually equal to its arc length further correction was applied ad-infinitum to each of the terms of the value obtained for jivā and sāra which ultimately give rise to equation 1 and 2