不用对称设法也可。。a(n) = 1/3 + (n-1)d,4 = a(2)+a(5) = 1/3 + d + 1/3 + 4d = 2/3 + 5d, d = 2/3.a(n) = 1/3 + 2(n-1)/3,33 = 1/3 + 2(n-1)/3,99 = 1 + 2(n-1),n = 98/2 + 1 = 50
