Monday, April 15, 2013

Solve for real x equation lgx+lg(9-2x)=1.

We'll impose the constraints of existence of the
logarithms:


x>0


9-2x>0
=> -2x>-9 => x < 9/2


The common
interval of admissible values is (0 ; 9/2).


We'll solve the
equation using the product rule of logarithms:


lg x*(9-2x)
= 1


We'll take
antilogarithm:


x*(9-2x) =
10


We'll remove the
brackets:


9x - 2x^2 = 10


We'll
move all terms to the left:


-2x^2 + 9x - 10 =
0


2x^2 - 9x + 10 = 0


We'll
apply quadratic formula:


x1 = [9+sqrt(81 -
80)]/4


x1 = (9+1)/4


x1 =
5/2


x2 = 8/4


x2 =
2


Since both x values belong to the interval
(0 ; 9/2), we'll accept them as solutions: {2 ;
5/2}.

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