已知An为等差数列，若a1＋a5+a9=8π.则cos(a2+a8)的值
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a1 + a5 + a9 = 3 a5 = 8π
得a5 = 8π/3
a2 + a8 = 2 a5 = 16π/3
cos(a2 + a8) = cos(16π/3) = cos(2π/3) = -1/2
得a5 = 8π/3
a2 + a8 = 2 a5 = 16π/3
cos(a2 + a8) = cos(16π/3) = cos(2π/3) = -1/2
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a(n) = a + (n-1)d,
8PI = a(1) + a(5) + a(9) = a + a+4d + a+8d = 3a + 12d = 3(a+4d),
(a+4d) = 8PI/3.
a(2) + a(8) = a+d + a+7d = 2a + 8d = 2(a+4d) = 2*8PI/3 = 16PI/3 = 5PI + PI/3,
cos[a(2)+a(8)] = cos[5PI + PI/3] = cos[PI + PI/3] = -cos[PI/3] = -1/2
8PI = a(1) + a(5) + a(9) = a + a+4d + a+8d = 3a + 12d = 3(a+4d),
(a+4d) = 8PI/3.
a(2) + a(8) = a+d + a+7d = 2a + 8d = 2(a+4d) = 2*8PI/3 = 16PI/3 = 5PI + PI/3,
cos[a(2)+a(8)] = cos[5PI + PI/3] = cos[PI + PI/3] = -cos[PI/3] = -1/2
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