Решение.
15. (b + c)(b - c) - b(b - 2c) = b^2 - c^2 - b^2 + 2bc = -c^2 + 2bc
16. (a - c)(a + c) - c(3a - c) = a^2 - c^2 - 3ac + c^2 = a^2 - 3ac
17. a(a + 5b) - (a + b)(a - b) = a^2 + 5ab - (a^2-b^2) = a^2 + 5ab - a^2 + b^2 = 5ab + b^2
18. b(3a - b) - (a - b)(a + b) = 3ab - b^2 - (a^2-b^2) = 3ab - b^2 - a^2 + b^2 = 3ab - a^2
19. (y + 10)(y - 2) - 4y(2 - 3y) = y^2 - 2y + 10y - 20 - 8y + 12y^2 = 13y^2 - 20
20. (a - 4)(a + 9) - 5a(1 - 2a) = a^2 + 9a - 4a - 36 - 5a + 10a^2 = 11a^2 - 36
21. (2b - 3)(3b +2) - 3b(2b+3) = 6b^2 + 4b - 9b - 6 - 6b^2 - 9b = -14b - 6
22. (3a - 1)(2a - 3) - 2a(3a + 5) = 6a^2 - 9a - 2a + 3 - 6a^2 - 10a = -21a + 3
23. (m + 3)^2 - (m - 2)(m + 2) = m^2 + 6m + 9 - (m^2 - 4) = m^2 + 6m + 9 - m^2 +4 = 6m + 13
24. (a - 1)^2 - (a + 1)(a - 2) = a^2 - 2a + 1 - (a^2 - 2a + a - 2) = a^2 - 2a + 1 - a^2 + a + 2 = -a + 3
25. (c + 2)(c - 3) - (c - 1)^2 = c^2 - 3c + 2c - 6 - (c^2 - 2c + 1) = c^2 - 3c + 2c - 6 - c^2 + 2c - 1 = c - 7
26. (y - 4)(y + 4) - (y - 3)^2 = y^2 - 16 - (y^2 - 6y + 9) = y^2 - 16 - y^2 + 6y - 9 = -25 + 6y
27. (a - 2)(a + 4) - (a + 1)^2 = a^2 + 4a - 2a - 8 - (a^2 +2a + 1) = a^2 + 4a - 2a - 8 - a^2 - 2a - 1 = -9
28. (b - 4)(b + 2) - (b - 1)^2 = b^2 + 2b - 4b - 8 - (b^2 - 2b + 1) = b^2 + 2b - 4b - 8 - b^2 + 2b - 1 = 0 - 9 = -9
1/3х+1/4х=1-1/5
7/12х=4/5
х=4/5/7/12
х=4/5*12/7
х=48/35
х=1 13/35