b) `(2/5)^x + (2/5)^(x-2) = 29/4`
`⇒ (2/5)^x + (2/5)^x : (2/5)^2 = 29/4`
`⇒ (2/5)^x + (2/5)^x : 4/25 = 29/4`
`⇒ (2/5)^x + (2/5)^x . 25/4 = 29/4`
`⇒ (2/5)^x . (1 + 25/4) = 29/4`
`⇒ (2/5)^x . 29/4 = 29/4`
`⇒ (2/5)^x = 29/4 : 29/4`
`⇒ (2/5)^x = 1`
`⇒ x = 0`
c) `5^((x+3).(2x-4)) =1`
`⇒ (x+3).(2x-4) = 0`
`⇒ (x+3) = 0` hoặc `(2x - 4) = 0`
`⇒ x = -3` hoặc `x = 2`