Question
Solve the equation
\dfrac{1}{x^2}+\dfrac{1}{(x+2)^2}=\dfrac{10}{9}
Solve the equation
\dfrac{1}{x^2}+\dfrac{1}{(x+2)^2}=\dfrac{10}{9}
Let
Substitute to original equation to get rid of fraction. We get a new equation.
In the meantime, take reciprocal of (1) and (2)
\dfrac{1}{a}=x
\dfrac{1}{b}=x+2
Subtraction of them gives
\dfrac{1}{a}-\dfrac{1}{b} = -2
Simplifying the equation gives
From (3), the following equation is valid after subtraction of 2ab from both sides
a^2+b^2-2ab=\dfrac{10}{9} -2ab
(a-b)^2 = \dfrac{10}{9} -2ab
Replacing ab with (4), we get a quadratic equation in terms of a-b
(a-b)^2 = \dfrac{10}{9} -(a-b)
Let p = a-b. The equation is simplified as
p^2+p- \dfrac{10}{9}=0
Solving the quadratic equation, we get two solutions
p_1=-\dfrac{5}{3}, p_2=\dfrac{2}{3}
When p = -\dfrac{5}{3}, let's add 2ab in both sides of (3)
a^2+b^2+2ab=\dfrac{10}{9} +2ab
(a+b)^2=\dfrac{10}{9} +2ab
Replace ab with the result of (4)
Since a-b=p= -\dfrac{5}{3}
(a+b)^2=\dfrac{10}{9} -\dfrac{5}{3}=-\dfrac{5}{9}<0
which is impossible. Therefore, p = -\dfrac{5}{3} is cancelled
When p = \dfrac{2}{3}, that is a-b = \dfrac{2}{3} substitute to (5)
(a+b)^2=\dfrac{10}{9} + \dfrac{2}{3} =\dfrac{16}{9}
Taking square root of both sides gives
a+b = \pm\dfrac{4}{3}
Plus a-b = \dfrac{2}{3}, we get two equation systems.
\begin{cases} a + b =\dfrac{4}{3} \\ a - b = \dfrac{2}{3} \end{cases}
and
\begin{cases} a + b =-\dfrac{4}{3} \\ a - b = \dfrac{2}{3} \end{cases}
Solving the first equation system, we get a = 1, and then x = 1
Solving the second equation system, we get a = -\dfrac{1}{3} , and then x =-3
The graph of y=\dfrac{1}{x^2}+\dfrac{1}{(x+2)^2}-\dfrac{10}{9} illustrates the points when y =0