Rectangles in rectangles

Python 2, 66 59 bytes

lambda n,k:sum(a%n*(n-a%n)==a/n*(k-a/n)for a in range(n*k))

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Each possible rectangle inside the \$n \times k\$-rectangle can be specified by two integers, \$0 \le a \lt n\$ and \$0 \le b \lt k\$:

To verify a rectangle given \$a\$ and \$b\$, it suffices to check if one angle is a right angle. To do this I take the dot product of \$\binom{b}{0}-\binom{0}{a}=\binom{-b}{a}\$ and \$\binom{k-b}{n}-\binom{0}{a}=\binom{k-b}{n-a}\$ to check whether the angle at \$\binom{0}{a}\$ is a right angle:

$$ \langle \left( \begin{matrix} -b \\ a \\ \end{matrix}\right), \left(\begin{matrix} k-b \\ n-a \\ \end{matrix} \right) \rangle = 0 \\\Leftrightarrow a\cdot(n-a)-b\cdot(k-b)=0 \\\Leftrightarrow a\cdot(n-a)=b\cdot(k-b) $$

05AB1E, 10 8 bytes

LDI-*`¢O

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Commented:

          # implicit input: [n, k]
L         # for both values take the [1..x] range
          #   [[1,...,n], [1,...,k]]
 D        # duplicate this list
  I       # push the input [n,k]
   -      # subtract this from the ranges
          #   [[1-n,...,n-n], [1-k,...,k-k]]
          #  =[[-n+1,...,0], [-k+1,...,0]]
    *     # multiply with the ranges
          #   [[1*(-n+1),...,n*0], [1*(-k+1),...,k*0]]
     `    # push all lists of this list on the stack
      ¢   # count the occurences of each value of one list in the other
       O  # sum those counts

C (gcc), 63 61 bytes

Saved 2 thanks to ceilingcat!!!

s;a;f(n,k){for(s=a=n*k;a--;)s-=a%n*(n-a%n)!=a/n*(k-a/n);a=s;}

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Port of ovs's Python answer.