等比数列：a,ak,ak^2,ak^3等差数列：a-4,ak-21,ak^2-29,ak^3-1(ak^3-1) - (ak^2-29) = (ak^2-29) - (ak-21)(ak^2-29) - (ak-21) = (ak-21) - (a-4)ak^3 - 2*ak^2 + ak + 36 = 0ak^2 - 2*ak + a + 9 = 0k*(ak^2 - 2*a...