Explain why (cos ((theta)) ? + (sin ((theta)) 2 = 1

Imagine a right triangle where a and b are the legs and c is the hypothenuse.

Pitagora: a²+b²=c²

Divide by c

(a/c) ² + (b/c) ²=1

But a/c=sin (B) and b/c=cos (B)

Thus sin² + cos² = 1