\({y_1} = \frac{{{u^2}{{\sin }^2}\theta }}{{2g}}\) and \({y_2} = \frac{{{u^2}{{\cos }^2}\theta }}{{2g}}\)
\(\therefore {y_1} + {y_2} = \frac{{{u^2}{{\sin }^2}\theta }}{{2g}} + \frac{{{u^2}{{\cos }^2}\theta }}{{2g}}\)
\(\therefore {y_1} + {y_2} = \frac{{{u^2}}}{{2g}}\)