質問: expand ( 1 − 1 a ) 3 - expansion of (a - b)3 expand `(1 - 1/a)^3`
回答: `(1 - 1/a)^3` = `(1)^3 - (1/a)^3 - 3 xx (1)^2 xx 1/a + 3 xx 1 xx (1/a)^2` = `1 - 1/a^3 - 3/a + 3/a^2`