ln a right angled triangle ABC,right angled at B and let angle BCA be α.According to first identity of trigonometry sin²α+cos²α=1 ln the triangle ABC sinα=AB/AC (sinα=opposite side/hypotenuse) cosα=BC/AC (cosα=adjacent side/hypotenuse) Substituting these in the first equation we get AB²/AC² + BC²/AC² = 1 AB²+BC² = AC²
