Here is an example.
Graphics 800,600,0,2 x1#=50 y1#=50 x2#=150 y2#=50 x3#=70 y3#=350 SetBuffer BackBuffer() Color 255,255,255 While Not KeyHit(1) Cls Line x1,y1,x2,y2 Line x3,y3,x2,y2 Line x1,y1,x3,y3 x#=MouseX() y#=MouseY() If IsInTriangle(x,y,x1,y1,x2,y2,x3,y3) Text 10,10,"Inside" Else Text 10,10,"-" EndIf Flip Wend End ; by Charles H. Giffen from ; <a href="http://groups.google.com/groups?selm=3784B03B.F1CCF05D%40virginia.edu&oe=UTF-8&output=gplain" target="_blank">http://groups.google.com/groups?selm=3784B03B.F1CCF05D%40virginia.edu&oe=UTF-8&output=gplain</a> ; Blitz Port by Peter Scheutz Function IsInTriangle ( px#,py#, ax#,ay#,bx#,by#,cx#,cy# ) Local bc#,ca#,ab#,ap#,bp#,cp#,abc# bc# = bx*cy - by*cx ca# = cx*ay - cy*ax ab# = ax*by - ay*bx ap# = ax*py - ay*px bp# = bx*py - by*px cp# = cx*py - cy*px abc# = Sgn(bc + ca + ab) If (abc*(bc-bp+cp)>0) And (abc*(ca-cp+ap)>0) And (abc*(ab-ap+bp)>0) Then Return True End Function