Given: S is any point in its interior of ∆PQR.
To Prove: SQ + SR < PQ + PR
Construction: Produce QS to meet PR in T
Proof: In ∆PQT,
PQ + PT > QT
∵ The sum of the lengths of any two sides of a triangle is greater than the length of the third side ...(1)
⇒ PQ + PT > QS + ST ...(2)
| ∵ QT = QS + ST
In ∆SRT,
TR + ST > SR ...(3)
| ∵ The sum of the lengths of any two sides of a triangle is greater than the length of the third side
Adding (2) and (3), we get
PQ + PT + TR + ST > QS + ST + SR
⇒ PQ + PR + ST > QS + ST + SR
⇒ PQ + PR > SQ + SR
⇒ SQ + SR < PQ + PR