# Strange expres. for the deriv. of solid angle

zn53
hello
please see attached snapshot from the book of Ligthmann & al. (problem book in relativity and gravitation).
Can somebody please explain the expression for the derivative of the solid angle ?
here it is given as dvxdvy where the v's are speeds !!
How come this is so ?
Thanks,

#### Attachments

• solidangle.png
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