For perfectly inelastic collision, in the centre of mass(C.M) frame, final kinetic energy of the colliding system (both spheres) becomes zero. Hence initial kinetic energy of the system in C.M. frame completely turns into the internal energy (Q) of the formed body. Hence
Q=~Ti=12μ|→v1−→v2|2
Now from energy conservation ΔT=−Q=−12μ|→v1−→v2|2,
In lab frame the same result is obtained as ΔT=12(m1→v1+m2→v2)2m1+m2−12m1|→v1|2+m2|→v2|2
=−12μ|→v1−→v2|2