Start with the right-angled triangle
Start with the right-angled triangle

A right-angled triangle has one angle of 90∘90^\circ90∘, usually shown by a small square. The longest side, opposite the right angle, is always the hypotenuse.
Start with the right-angled triangle
Start with the right-angled triangle

A right-angled triangle has one angle of 90∘90^\circ90∘, usually shown by a small square. The longest side, opposite the right angle, is always the hypotenuse.