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A line perpendicular to one of two parallel lines is perpendicular to other.

Mark four points A, B, C and D on your notebook such that no three of them are collinear. Draw all lines joining these points in pairs.

How many such lines can be drawn?  

Mark four points A, B, C and D on your notebook such that no three of them are collinear. Draw all lines joining these points in pairs.

How many such lines can be drawn?  

If A, B and C are collinear, C, D and E are collinear, then A, B, C, D and E are collinear points.

In the figure, $\angle \mathrm{ACX}$ is an exterior angle of $△\mathrm{ABC}$. Name the interior opposite angles.

In the figure, $\angle \mathrm{ACX}$ is an exterior angle of $△\mathrm{ABC}$. Name the interior adjacent angles.

In a triangle, an exterior angle at a vertex is $105°$ and it's one of the interior opposite angles is $45°$. Find all angles of the triangle

The exterior angles obtained on producing the base of a triangle both ways are $96°$ and $128°$. Find all the angles of the triangle.