[tex] {a}^{2} + {b}^{2} + {c}^{2} = 21[/tex][tex] a + b - c = 7[/tex][tex]ab - bc - ac =[/tex]?

Question

Алгебра: [tex] {a}^{2} + {b}^{2} + {c}^{2} = 21[/tex][tex] a + b - c = 7[/tex][tex]ab - bc - ac =[/tex]? ​

ShiroDark · Answer

a² + b² + c² = 21a + b - c = 7 (a + b - c)² = 7²[(a + b) - c]² = 49(a + b)² - 2 * (a + b) * c + c² = 49a² + 2ab + b² - 2ac - 2bc + c² = 49(a² + b² + c²) + 2ab - 2bc - 2ac = 4921 + 2(ab - bc - ac) = 492(ab - bc - ac) = 28ab - bc - ac = 14...

${a}^{2} + {b}^{2} + {c}^{2} = 21$ $a + b - c = 7$ $ab - bc - ac =$ ?

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${a}^{2} + {b}^{2} + {c}^{2} = 21$ $a + b - c = 7$ $ab - bc - ac =$ ?