Explicación paso a paso:
Triángulo A:
[tex] {x}^{2} = {6}^{2} + {8}^{2} \\ {x}^{2} = 36 + 64 \\ x = \sqrt{100} \\ x = 10cm[/tex]
Triángulo B:
[tex] {13}^{2} = {5}^{2} + {x}^{2} \\ 169 - 25 = {x}^{2} \\ x = \sqrt{144} \\ x = 12cm[/tex]
Triángulo C:
[tex] {20}^{2} = {16}^{2} + {x}^{2} \\ 400 = 256 + {x}^{2} \\ {x}^{2} = 400 - 256 \\ x = \sqrt{144} \\ x = 12cm[/tex]
Espero que te sirva
