Question

show that in a right angled triangle the hypotenuse is longest side

Answer 1

In a right angled triangle, the angle opposite To the hypotenuse is 90°, while other two angles are Always less than 90°. As you know that the side opposite to the largest angle is always the largest in a triangle.
Refer for proof in pic.

Answer 2

In the given triangle ABC,

Angle B = 90°

Hence Angle A + Angle C = 180°- 90°  ...(Sum of all angles of the triangle are 180°)

Therefore Angle B is the greater angle than Angle A and Angle C. [Angle A>Angle C ]  

We know that, Side opposite to greater angle is always longer.

Therefore, Side opposite to angle B is AC

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[AC > BC and AC > AB]

That is AC is the longest side..

[AC is the hypotenuse]

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