To solve the problem it is necessary to resort to the concepts of kinetic energy of the bodies.
Kinetic energy in vector form can be expressed as
[tex]KE = \frac{1}{2}m\vec{R}^2[/tex]
According to the description given we have to
[tex]r_1 = R+ m_2r/M[/tex]
[tex]r_2 = R-m_1 r/M[/tex]
Equating both equation we have that
[tex]R = \frac{m_1r_1+m_2r_2}{m_1+m_2}[/tex]
[tex]R = \frac{m_1r_1+m_2r_2}{M}[/tex]
The kinetic energy of the two particles would be given by
[tex]T = \frac{1}{2} (m_1\vec{r_1}^2+m_2\vec{r_2}^2)[/tex]
[tex]T = \frac{1}{2} (m_1(\vec{R}+\frac{m_2}{M}\vec{r})^2+m_2(\vec{R}-\frac{m_1}{M}\vec{r})^2)[/tex]
[tex]T = \frac{1}{2} (m\vec{R}^2+\frac{m_1m_2}{M}\vec{r}^2)[/tex]
We have the consideration that
[tex]\mu = \frac{m_1m_2}{M}[/tex]
Then replacing,
[tex]T = \frac{1}{2}(m\vec{R}^2+\mu\vec{r^2})[/tex]