(mathematics) arithmetic sequence, arithmetic progression

difficult / hard, probably impossible

untie; unravel; undo (a fastening); unwrap / dispel (doubts); clear up (a matter)

solve (a problem); answer (a question); unravel (a riddle) / comb; untangle (hair)

(mathematics) equal difference (between numbers)

(mathematics) a sequence of numbers, especially one that shows or is created under a certain pattern

question, problem on homework etc. / a question; a matter (of time etc.) / a problem (something that needs to be dealt with); a trouble / an issue; a matter

(mathematics) arithmetic series

(mathematics) geometric sequence

(mathematics) Fibonacci sequence