7.9. :
a) a + (b - a) = (a + b); ) a - (a + b) = (a - b).
7.10. - ABC. .
7.11. M - ABC, O - . .
7.12. E F - AD BC ABCD. , . .
7.13. m n. : a = 2 m - 3 n, b = 3 m + n, c = 6 m + 2 n. : ) a b, ) a c, ) b c.
7.14. l, m, n, a, b, c: l = a + b - c, m = b, n = a + 2 b - c.
7.15. a, b c. ,
a + 2 b - c, 3 a - b + c, - a + 5 b - 3 c .
7.16. a, b c. l m, l a + m b + c a + l b + m c .