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Section II. Geometry of Determinants 325<br />

1.28 This question uses material from the optional Determinant Functions Exist<br />

subsection. Prove Theorem 1.5 by using the permutation expansion formula for<br />

the determinant.<br />

� 1.29 (a) Show that this gives the equation of a line in R 2 thru (x2,y2)and(x3,y3).<br />

� �<br />

�x<br />

x2 x3�<br />

� �<br />

�y<br />

y2 y3�<br />

�<br />

1 1 1<br />

� =0<br />

(b) [Petersen] Prove that the area of a triangle with vertices (x1,y1), (x2,y2),<br />

and (x3,y3) is<br />

�<br />

�x1<br />

1 �<br />

�y1<br />

2 �<br />

1<br />

x2<br />

y2<br />

1<br />

�<br />

x3�<br />

�<br />

y3�<br />

1<br />

� .<br />

(c) [Math. Mag., Jan. 1973] Prove that the area of a triangle with vertices at<br />

(x1,y1), (x2,y2), and (x3,y3) whose coordinates are integers has an area of N<br />

or N/2 for some positive integer N.

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