\(\begin{array}{l} \text{Xét }\Delta ABC\text{ vuông tại }A\text{ ta có:}\\ \sin \widehat B = \dfrac{{AC}}{{BC}};\,\,\,\cos \widehat B = \dfrac{{AB}}{{BC}}\\ \Rightarrow {\sin ^2}B + {\cos ^2}B = {\left( {\dfrac{{AC}}{{BC}}} \right)^2} + {\left( {\dfrac{{AB}}{{BC}}} \right)^2} = \dfrac{{B{C^2}}}{{B{C^2}}} = 1.\\ \Rightarrow {\sin ^2}x + {\cos ^2}x = 1. \end{array}\).
