determine whether point lies inside triangle?

Question

The program needs to read the values of three coordinates - P1(x1,y1) - P2(x2,y2) - P3(x3,y3) as well as another coordinate P(x,y) and determine whether this point is inside a triangle formed from the 3 point above.

Answer 1

Draw a vertical raw UPWARD from the point, sufficiently long so that it is surely to clear the triangle. If the line crosses the triangle at one point, then the point is inside the triangle.