In parallelogram PQRS, P(-3,2), Q(2,7) and S(1,9) are three vertices. Then the length of diagonal PR is
Draw PE, SF parallel to x-axis.
Draw QE, RF parallel to y-axis.
Now, in right triangles △PEQ and △SFR,
PQ = SR
∠QPE = ∠RSF
∠PQE = ∠SRF
Therefore, PE = SF and QE = RF .....[by RHS congruency]
Hence, PE = SF = 2 - (-3) = 5 and QE = RF = 7 - 2 = 5
Now, x-coordinate of R = 1 + SF
= 1 + 5
= 6
y-coordinate of R = 9 + RF
= 9 + 5
= 14
Therefore coordinates of R(6,14).
∵ The distance between two points (x1,y1) and (x2,y2) is √(x2−x1)2+(y2−y1)2
∴ PR = √(6−(−3)2+(14−2)2
= √92+122
= √225
= 15 units