Cross product to find the area of a triangle

molly1103
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okay so I know that the area of the triangle is half the area of the parallelogram, ill try using pictures because this is a bit confusing to describe only with words:
for example we have this
http://farside.ph.utexas.edu/teaching/301/lectures/img243.png
and then if we use the cross product of a and b and we find the magnitude of that vector and we divide it by 2 we get the area of the triangle with vertices ADB, so far so good. but i am confused as to how do we find the area of the triangle ADC? my professor said that we can't find it simply by finding the cross product and dividing it by two... anybody can help me with this? :(
thanks!
 
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Look at the two triangles ABD and ACD. Geometrically, how would you calculate their areas? Are these areas different?
 

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