In △PQR, ∠P = 70° and ∠R = 30°. Which side of this triangle is longest? Give reason for your answer.

We know that,

Sum of angles of triangle = 180°.

∴ ∠P + ∠Q + ∠R = 180°

⇒ 70° + ∠Q + 30° = 180°

⇒ ∠Q + 100° = 180°

⇒ ∠Q = 80°.

We know that side opposite to the greatest angle is greatest side.

∴ PR is the greatest side, which is opposite to ∠Q.

Hence, PR is the greatest side because ∠Q = 80° and side opposite to the greatest angle is greatest side.