x2[log(x)]² = 10x3
Take logs of both sides:
log{x2[log(x)]²} = log(10x3)
Use rules of logarithms:
2[log(x)]²·log(x) = log(10) + log(x³)
2[log(x)]³ = 1 + 3·log(x)
2[log(x)]³ - 3·log(x) - 1 = 0
let u = log(x)
2u³ - 3u - 1 = 0
Possible rational solutions for u are ±1, ±
Try 1:
1|2 0 -3 -1
| 2 2 -1
2 2 -1 -2
No that isn't a solution, since it did not give a remainder of 0.
Try -1:
-1|2 0 -3 -1
| -2 2 1
2 -2 -1 0
Yes -1 is a solution, so we have factored
2u³ - 2u - 1 = 0
as
(u + 1)(2u² - 2u - 1) = 0
u + 1 = 0 2u² - 2u - 1 = 0
u = -1 u =
u =
u =
u =
u =
u =
u =
The log equation u = log(x) is equivalent to the exponential
equation x = 10u
So we have three solutions:
x = 10-1, x =
, x =
In decimal approximations they are
x = 0.1, x = 23.22872667, x = 0.4305014278
Edwin