a b c d

b

Output of upper AND gate $=\$\backslash overline\{A\}B\$$ Output of lower AND gate $=\$\backslash overline\{B\}A\$$ Thus, output of OR gate $=\$\backslash overline\{A\}B\$\; +\; \$\backslash overline\{B\}A\$$ $\text{This is Boolean expression for XOR gate.}$