See the attached image for the answers
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Problem 17
The fact that we have perpendicular diagonals means we either have a kite or a rhombus. We can't have a kite because the opposite sides are parallel. So that means the figure is a rhombus.
An alternative line of thinking is to break the figure into 4 triangles, and then prove each triangle to be congruent. This will show that all four sides are the same length (by CPCTC). So that's another way to see we have a rhombus.
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Problem 18
A rectangle has congruent diagonals. Draw in the diagonals and then prove the triangles to be congruent using SAS. By CPCTC, the diagonals will be the same length.
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Problem 19
RP and VA are the first two letters of RPQ and VAQ respectively
RP/VA = 6/3 = 2/1
Similarly,
PQ/AQ = 8/4 = 2/1
The triangles are similar
The scale factor is 2:1