Question

数学题(log)

Answer 1

更新1:

有少少野想问@@ 第1题第2步点解无左个log既[email protected]@ 第2题第3题点解X>12同X>0既@@ 小弟岩岩先学log
好多唔明...还请见谅@@

log8(X - 5) = 2/3 82/3 = X - 5 4 = X - 5 X = 9 log(X + 3) + log(X + 12) - 1 = 0 ; X > -12 log(X + 3) + log(X + 12) = 1 log(X + 3)(X + 12) = 1 (X + 3)(X + 12) = 10 X2 + 15X + 36 = 10 X2 + 15X + 26 = 0 (X + 13)(X + 2) = 0 X = -13(rejected) or X = -2 (logX)2 - logX2 = 3 (logX)2 - 2logX - 3 = 0 Let logX = u; X>0 u2 - 2u - 3 = 0 (u - 3)(u + 1) = 0 u = 3 or u = -1(rejected) logX = 3 X = 103 = 1000 2009-09-14 19:03:38 补充： Correction: -1 is not rejected logX = -1 X = 10^-1 = 0.1 2009-09-14 19:06:23 补充： Just too quick 2009-09-14 19:37:50 补充： log(a)b = x <=> b^x = a这是定义
要记. 如10^4 = 10000
log10000 =4 5^3 = 125
log(5)125 = 3 logX = Y => 10^Y = X
X一定是正数.
For Q 3
why u can't be -1? when u = -1
X = 0.1
still positive