Question

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M is the midpoint of 0A. N is the midpoint of OB. Prove that AB is parallel to MN​

Answer 1

Construct MN.

Since M is the midpoint of OA, OM = MA

Similarly, N is the midpoint of OB.

Thus, ON = NB.

Now, in Δs OMN and OAB,

∠MON = ∠AOB (common angle)

(sides are in proportional ratio; OA = 2OM and OB = 2ON)

∴ Δs OMN and OAB are similar (2 sides are in proportion, with the included angle)

Since they are similar, then ∠OMN = ∠OAB (corresponding angles of similar triangles are equal)

But since ∠OMN = ∠OAB, then that means MN || AB (corresponding angles of two lines must be equal since they also sit relative to the transverse line, OA)

Thus, AB || MN (QED)