Đáp án:
`a,`
`25^3 ÷ 5^2`
`= (5^2)^3 ÷ 5^2`
`= 5^6 ÷ 5^2`
`= 5^4`
$\\$
`b,`
`(3/7)^{21} ÷ (9/49)^6`
`= (3/7)^{21} ÷ [(3/7)^2]^6`
`= (3/7)^{21} ÷ (3/7)^{12}`
`= (3/7)^{9}`
$\\$
`c,`
`3 - (-6/7)^0 + (1/2)^2 ÷ 2`
`3 - 1 + 1/4 × 1/2`
`= 3 - 1 + 1/8`
`= 2 + 1/8`
`= 16/8 + 1/8`
`= 17/8`