0\\\\sin^2a+cos^2a=1\; \; \to \; \; cosa=+\sqrt{1-sin^2a}=\sqrt{1-\frac{25\cdot 3}{196}}=\\\\=\sqrt{\frac{121}{196}}=\frac{11}{14} " alt=" sina=\frac{5\sqrt3}{14}\\\\a\in (0;\frac{\pi }{2})\; \; \to \; \; cosa>0\\\\sin^2a+cos^2a=1\; \; \to \; \; cosa=+\sqrt{1-sin^2a}=\sqrt{1-\frac{25\cdot 3}{196}}=\\\\=\sqrt{\frac{121}{196}}=\frac{11}{14} " align="absmiddle" class="latex-formula">