Because it is true for 1, then it is true for 1+1 (=2, by the induction step), then it is true for 2+1 (=3), then it is true for 3+1 (=4), and so on.
b ɪ k ʌ z WORD_BOUNDARY ɪ t WORD_BOUNDARY ɪ z WORD_BOUNDARY t ɹ uː WORD_BOUNDARY f ɔ ɹ WORD_BOUNDARY w ʌ n WORD_BOUNDARY ð ɛ n WORD_BOUNDARY ɪ t WORD_BOUNDARY ɪ z WORD_BOUNDARY t ɹ uː WORD_BOUNDARY f ɔ ɹ WORD_BOUNDARY w ʌ n WORD_BOUNDARY p l ʌ s WORD_BOUNDARY w ʌ n WORD_BOUNDARY iː k w ə l z WORD_BOUNDARY t uː WORD_BOUNDARY b aɪ WORD_BOUNDARY ð ɪ WORD_BOUNDARY ɪ n d ʌ k ʃ ə n WORD_BOUNDARY s t ɛ p WORD_BOUNDARY ð ɛ n WORD_BOUNDARY ɪ t WORD_BOUNDARY ɪ z WORD_BOUNDARY t ɹ uː WORD_BOUNDARY f ɔ ɹ WORD_BOUNDARY t uː WORD_BOUNDARY p l ʌ s WORD_BOUNDARY w ʌ n WORD_BOUNDARY iː k w ə l z WORD_BOUNDARY θ ɹ iː WORD_BOUNDARY ð ɛ n WORD_BOUNDARY ɪ t WORD_BOUNDARY ɪ z WORD_BOUNDARY t ɹ uː WORD_BOUNDARY f ɔ ɹ WORD_BOUNDARY θ ɹ iː WORD_BOUNDARY p l ʌ s WORD_BOUNDARY w ʌ n WORD_BOUNDARY iː k w ə l z WORD_BOUNDARY f ɔ ɹ WORD_BOUNDARY æ n d WORD_BOUNDARY s oʊ WORD_BOUNDARY ɔ n WORD_BOUNDARY