How old is "Zaken"?
Radak: It is 70 years, for he was 30 when he became king, and he was king for 40 years (Shmuel II, 5:4).
What is the meaning of "Ba ba'Yamim"?
Radak: In his old age, he became weak and bedridden. Even though he was only 70, due to the wars he fought old age sprang upon him. When one ages, natural body heat decreases.
Why didn't garments help David to get warm?
Rashi citing Brachos 62b: Anyone who disgraces clothing, in the end he will not benefit from clothing, David cut the corner 1 of Sha'ul's garment (Shmuel I, 24:5).
Rashi citing Tana d'Vei Eliyahu Rabah 7: When David saw the angel standing in Yerushalayim with his sword drawn, his blood was chilled due to fear. Radak - "Ki Niv'as Mipnei ha'Mal'ach" (Divrei ha'Yamim I, 21:30).
Radak: When one ages, natural body heat decreases. Malbim - garments do not give warm. They only stop the air from cooling off the body's own warmth!
Iyun Yakov (62b): Even though he cut it to convince Sha'ul not to pursue him, he did not have to cut the entire corner. Ben Yehoyada - there were Tzitzis on it; he could have cut a different part to avoid invalidating the Mitzvah.
What is the meaning of "v'Lo Yecham Lo"?
Radak: His flesh did not become warm. This is like "va'Yecham Besar ha'Yeled" (Melachim II, 4:34) and 'v'Cham l'Adoni ha'Melech" (verse 2). The root is Yacham; the Yud in Yecham is of the conjugation; the Yud of the root is omitted. The same applies to "va'Yira Sha'ul mi'Lifnei David" (Shmuel I, 18:12), "va'Yirash Zar'acha" (Bereishis 22:17) and similar words.
Sefer Shmuel discusses what happened to David. Why is this episode not there?
Malbim (1-2, citing Mahari): This Sefer discusses all matters of Shlomo and his anointment, including Adoniyahu's rebellion. Adoniyahu tried to make himself king without David's knowledge. This prompted David to anoint Shlomo that day, even though normally a Melech Ben Melech need not be anointed (Horiyos 11b). Why did Adoniyahu try to seize power in David's lifetime? Did he not realize that this will arouse David's anger, and spur him to anoint Shlomo? It was because David strength waned and he ceased to rule, and lied on a bed covered with garments 1 . He was alone with Avishag; no one would enter to tell him that Adoniyahu seized power. Also refer to 2:17:2:3.
Garments did not warm him! Perhaps this was initially, before he realized that garments will not help. (PF)
