ऋषिभिर्बहुधा गीतं छन्दोभिर्विविधैः पृथक्।
13.5. This has been sung many times by sages, and also has been clearly decided in the various Vedas in different contexts by means of [their] words that are suggestive of the Brahman (i.e. in the Upanisads) and are full of reasoning.
13.4-5 Tat Ksetram etc. Rsibhih etc. Why it modifies : due to what this [Field] suffers modification. Collectively : not at all separately (one by one). [The Bhagavat] decides all the estions in a general way. Of course, many a time in many a way this has been declared by the seers and by the scritpures. But, let Me (the Bhagavat) explain this collectively (briefly).
Rishibhirbahudhaa geetam cchandobhirvividhaih prithak; Brahmasootrapadaishchaiva hetumadbhirvinishchitaih.
ṛiṣhibhiḥ—by great sages; bahudhā—in manifold ways; gītam—sung; chhandobhiḥ—in Vedic hymns; vividhaiḥ—various; pṛithak—variously; brahma-sūtra—the Brahma Sūtra; padaiḥ—by the hymns; cha—and; eva—especially; hetu-madbhiḥ—with logic; viniśhchitaiḥ—conclusive evidence