Answer:
Pythagorean theorem
Step-by-step explanation:
We know that for a triangle if we know two sides and the included angle, the third side can be found out using cosine formula
\(a^2=b^2+c^2-2bc cos A\)
For a right angle triangle right angled at A this becomes
\(a^2=b^2+c^2-2bc cos A\\=b^2+c^2-2bc cos 90\\=b^2+c^2-0\\=b^2+c^2\)
This is nothing but Pythagorean theorem
