The system of linear equations

5x + ky = 5,

3x + 3y = 5; will be consistent if

The equation of the tangent to the curve y (1 + x²) = 2 $$-$$ x, where it crosses the x-axis is

If $$\left[ {\matrix{ {3c + 6} & {a - d} \cr {a + d} & {2 - 3b} \cr } } \right] = \left[ {\matrix{ {12} & 2 \cr { - 8} & { - 4} \cr } } \right]$$ are equal, then value of ab $-$$ cd is

The principal value of $${\tan ^{ - 1}}\left( {\tan {{9\pi } \over 8}} \right)$$ is