.
C = 13
B = 12
What does A =?
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Since your question goes under the section "Pythagorean Theorem",
I can conclude that your question is about a right-angled triangle.
Standard naming for the right-angled triangle sides is: A and B are the legs, and C is the hypotenuse.
And the Pythagorean Theorem says: = , or
A = .
Now substitute your data, and you will learn what A does:
A = = = = 5.
Answer. A = 5. // A does 5.
Had you formulate your question in more clear way, you'd got the clear answer faster.
My gift for you are these two lessons that talk about the Pythagorean theorem:
The Pythagorean Theorem
More proofs of the Pythagorean Theorem