How Sin²θ + Cos²θ = 1?

How

 Sin²θ + Cos²θ = 1...??

According to Pythagoras theorem,

"the square of hypotenuse(H) is equal to the sum of square of base(B) and perpendicular(P)"

i.e.     H² = B² + P²

(Dividing both sides by H²)

we get,

    H²/H² = B²/H² + P²/H²

       1   = Sin²θ + Cos²θ    

                  ....(B/H  = sinθ and P/H = cosθ)

Hence, Sin²θ + Cos²θ = 1

How Sin²θ + Cos²θ = 1?