Saturday, November 6, 2010

What are the real solutions of the equation? log2(x+1)+log2(x-2)=2

Since the bases of the logarithms are matching, we'll
apply the product property:


log 2(x+1)+log2(x-2)=log2
[(x+1)(x-2)]


We'll re-write the
equation:


log2 [(x+1)(x-2)] =
2


We'll take
antilogarithms:


[(x+1)(x-2)] =
2^2


[(x+1)(x-2)] = 4


We'll
remove the brakets:


x^2 - x - 2 - 4 =
0


x^2 - x - 6 = 0


We'll apply
quadratic formula:


x1 = [1+sqrt(1 +
24)]/2


x1 = (1 + 5)/2


x1 =
3


x2 = -2


Since
the common interval of admissible values for x (values that make the logarithms to
exist) is (2 , +infinite), we'll reject the negative value and we'll keep as solution of
equation only x = 3.

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