There are 18 points in a plane of which 5 are collinear. How many straight lines can be formed by joining them?

Given: 18 points are their in a plane, of which 5 are collinear.

To find: No's of straight line can be formed.

Jo form a straight line we need at least 2 points.

Therefore, ${ }^{18} c_{2}$

Now, 5 points are collinear. hence it will form a line

$\therefore \quad{ }^{5} C_{2}$

Hence, $\quad{ }^{18} C_{2}-{ }^{5} C_{2}+1 \quad[+1$ is added for 5 collinear points]

$\begin{array}{l} =\frac{18!}{2!16!}-\frac{5!}{2! 3!}+1 \\ =153-10+1 \\ =144 \end{array}$

Therefore total 144 line will be formed.