Đáp án đúng: B
Giải chi tiết:\(\begin{array}{l}f\left( x \right) = \frac{{{9^x}}}{{{9^x} + 3}}\\f\left( a \right) = \frac{{{9^a}}}{{{9^a} + 3}}\\f\left( b \right) = f\left( {1 - a} \right) = \frac{{{9^{1 - a}}}}{{{9^{1 - a}} + 3}}\\f\left( a \right) + f\left( b \right) = \frac{{{3^{2a}}}}{{{3^{2a}} + 3}} + \frac{{{3^{2 - 2a}}}}{{{3^{2 - 2a}} + 3}} = \frac{{{3^{2a}}}}{{{3^{2a}} + 3}} + \frac{{\frac{{{3^2}}}{{{3^{2a}}}}}}{{\frac{{{3^2}}}{{{3^{2a}}}} + 3}}\\\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = \frac{{{3^{2a}}}}{{{3^{2a}} + 3}} + \frac{{{3^2}}}{{{3^2} + {{3.3}^{2a}}}}\,\, = \frac{{{3^{2a}}}}{{{3^{2a}} + 3}} + \frac{3}{{3 + {3^{2a}}}} = \frac{{{3^{2{\rm{a}}}} + 3}}{{3 + {3^{2a}}}} = 1.\end{array}\)
Chọn B.
