The number of n-digit numbers,no two consecutive digits being the same is

A reserve of 12 railway station masters is to be divided into two groups of 6 each one for day duty and the other for night duty. The number of ways in which this can be done if two specified persons A,B should not be included in the same group is

For such questions, we should know different rules and formulae of limits.

The value of  if is

For such questions, we have to remember the different formulas of limit.

For such questions, we should remember the formulae of limit.

 where  and 

For such questions, we should know different formulas of limit.

Let PQ and RS be tangents at the extremities of the diameter ‘PR’ of a circle of radius ‘r’. If PS and RQ intersect at a point ‘X’ on the circumference of the circle, then 2r equals :

For such questions, we should be know different formulas of limit.

If and are two tangents to a circles then radius of the circle is

The locus of center of a circle which passes through the origin and cuts off a length of 4 units from the lineis: