See here https://gre.myprepclub.com/forum/gre-math- ... -2609.html

Geometry chapter

We can do it without the formula too!

Just plot the points on a rough co-ordinate system. You will notice that the base is 5 and height is 10
Base = (6 - 1) = 5
Height = 10 [vertical distance of point P from x-axis]

Area = $\frac{1}{2}bh = \frac{1}{2}(5)(10) = 25$

Hence, option A

what's the source?

Simply drop a perpendicular to the X axis from the point (14,10), the height of this perpendicular is the height of the triangle, 10 in this case. The base is the distance between the points (1,0) and (6,0) which is 5.

So Area = 1/2*5*10 = 25.

In cases where one of the sides of the triangle does not lie solely along the X or Y axes, I would simply extend the vertices of the relevant side to form a right angled triangle and employ the Pythagoras theorem to find the length of that side.