subtract squares both sides equations "momentum" from "energy" and use the identity cosh 2 ⁡ ⁡ ( s ) − − sinh 2 ⁡ ⁡ ( s ) = 1 , after simplifying we get: 2 m 1 m 2 ( cosh ⁡ ⁡ ( s 1 ) cosh ⁡ ⁡ ( s 2 ) − − sinh ⁡ ⁡ ( s 2 ) sinh ⁡ ⁡ ( s 1 ) ) = 2 m 1 m 2 ( cosh ⁡ ⁡ ( s 3 ) cosh ⁡ ⁡ ( s 4 ) − − sinh ⁡ ⁡ ( s 4 ) sinh ⁡ ⁡ ( s 3 ) )