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Exercise 7.5 · Q18

Q.If x1,x2,x3x_1, x_2, x_3 as well as y1,y2,y3y_1, y_2, y_3 are in geometric progression with the same common ratio, then the points (x1,y1), (x2,y2), (x3,y3)(x_1, y_1),\ (x_2, y_2),\ (x_3, y_3) are

(1) vertices of an equilateral triangle
(2) vertices of a right angled triangle
(3) vertices of a right angled isosceles triangle
(4) collinear
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Substitute the GP terms into the area determinant.

This tests collinearity via determinants.

Step 1. Let the common ratio be rr: x2=x1r, x3=x1r2x_2 = x_1 r,\ x_3 = x_1 r^2 and y2=y1r, y3=y1r2y_2 = y_1 r,\ y_3 = y_1 r^2.

Step 2. The area =12∣x1y11x1ry1r1x1r2y1r21∣= \tfrac{1}{2}\begin{vmatrix} x_1 & y_1 & 1 \\ x_1 r & y_1 r & 1 \\ x_1 r^2 & y_1 r^2 & 1 \end{vmatrix}. Compute the cross terms: (P2−P1)=(x1(r−1), y1(r−1))(P_2 - P_1) = (x_1(r-1),\ y_1(r-1)) and (P3−P1)=(x1(r2−1), y1(r2−1))(P_3 - P_1) = (x_1(r^2-1),\ y_1(r^2-1)) …

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