Respuesta :

Answer:

9

Step-by-step explanation:

Using the identity

(a + b + c)² = a² + b² + c² + 2(ab + bc + ca)

Subtract 2(ab + bc + ca) from both sides, then

a² + b² + c² = (a + b + c)² - 2(ab + bc + ca) = 7² - 2(20) = 49 - 40 = 9

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