Complex sentence: A curve "E" defined over the field of rational numbers is also defined over the field of real numbers, therefore the law of addition (of points with real coordinates) by the tangent and secant method can be applied to "E".
Split into simpler sentences: A curve "E" defined over the field of rational numbers is also defined over the field of real numbers. Therefore the law of addition (of points with real coordinates) by the tangent and secant method can be applied to "E".