Chọn B
y ' = 2 x . a + 2 x 2 + ( x 2 + 1 ) . 1 2 a 2 + x 2 . ( a 2 + x 2 ) ' a 2 + x 2 = 2 x . a + 2 x 2 + x ( x 2 + 1 ) a 2 + x 2 a 2 + x 2 = 2 x ( a 2 + x 2 ) + x ( x 2 + 1 ) ( a 2 + x 2 ) . a 2 + x 2 = x ( 3 x 2 + 2 a 2 + 1 ) ( a 2 + x 2 ) . a 2 + x 2