續二次方程(19,20題)(log)

19. 原式中有自然條件x>4
log[5](x+2) + log[5](x-4) = 1 + log[5](x)
log[5](x+2)(x-4) = log[5]5x
(x+2)(x-4) = 5x
x^2 - 7x - 8 = 0
(x-8)(x+1) = 0
x = 8 .......................答案OA O(1不合)

20. log[2](x^log[2](x)) = log[2](x^5) - 3log[2]4
log[2](x) * log[2](x) = 5log[2]x - 3 * 2
{ log[2](x) }^2 - 5 { log[2](x) } + 6 = 0
{ log[2](x) - 2 } * { log[2](x) -3 } = 0
log[2](x) = 2 or 3
x = 4 or 8 .......................答案OA O