Ответы на вопрос:
(b+c-2a)(c-b)+(c+a-2b)(a-+b-2c)(a-b)= =bc+c²-2ac-b²-bc+2ab+ac+a²-2ab-c²-ac+2bc-a²-ab+2ac+ab+b²-2bc= =(bc-bc+2bc-2bc)+(ac-ac+2ac-2ac)+(ab-ab+2ab-2ab)+(a²-a²+b²-b²+c²-c²)≡0.
a2= a1 + d
a3 = a1 + 2d
'''''
a10= a1 + 9d
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S(1-5) = a1 + a2 + a3 + a4 + a5 = a1 + a1 + d + a1 + 2d + a1 + 3d + a1 + 4d = 5a1 + 10d
S(6-10) = a1 + 5d + a1 + 6d + a1 + 7a + a1 + 8d + a1 + 9d = 5a1 + 35d
5a1 + 10d = 275
5a1 + 35d = 90
25d = -185
5d = -37
5a1 + 2*5d = 275
5a1 - 74 = 275
5a1 = 349
S(11-15) = a1 + 10d + a1 + 11d + a1 + 12d + a1 + 13d + a1 + 14d = 5a1 + 60d
S(11-15) = 5a1 + 60d = 5a1 + 12*5d = 349 + 12*(-37) = 349 - 444 = -95
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