How do you solve Log (x +3)(x-2) = Log (7x-11)log(x+3)(x2)=log(7x11)?

1 Answer
Nov 17, 2015

This equation has one solution x=5x=5

Explanation:

First we have to find the domain of this equation. Since loglog functions are only defined for positive real numbers we have to solve inequalities:

(x+3)(x-2)>0(x+3)(x2)>0 and 7x-11>07x11>0

From first inequality we get: x<-3 vv x>2x<3x>2

From the second we get: x> 1 4/7x>147

So we can write, that the domain is: D=(2;+oo)D=(2;+)

Now we can solve the equation:

log(x+3)(x-2)=log(7x-11)log(x+3)(x2)=log(7x11)

Since the base of logarythm is the same on both sides (none is written, so I suppose it is 1010), we can skip loglog signs and solve equation:

(x+3)(x-2)=7x-11(x+3)(x2)=7x11

x^2+x-6=7x-11x2+x6=7x11

x^2-6x+5=0x26x+5=0

Delta=6^2-4*1*5

Delta=36-20=16

sqrt(Delta)=4

x_1=(-b-sqrt(Delta))/(2a)

x_1=(6-4)/2=1

x_2=(-b+sqrt(Delta))/(2a)

x_2=(6+4)/2

x_2=5

Since x_1 !in D, the only solution of this equation is x_2=5