Question 18621
y=m1x+c1 meets x=0 at y=c1..y=m2x+c2 meets x=0 at y=c2
and y=m1x+c1 meets y=m2x+c2...at m1x+c1=m2x+c2..or...x(m1-m2)=(c2-c1)..or..
x= (c2-c1)/(m1-m2)and y=[m1(c2-c1)/(m1-m2)]+c1
the 3 vertices are..(0,c1),(0,c2)and (x1,y1)where x1=(c2-c1)/(m1-m2),y1=((m1(c2-c1)/(m1-m2))+c1
area={{{matrix(3,3,0,c1,1,0,c2,1,x1,y1,1)}}}*(1/2)                       
area=(1/2)*x1(c1-c2)=(1/2)*(c1-c2)*(c2-c1)/(m1-m2)