I see this so much in questions here and elsewhere...
when you come to post something, do not use "I have a doubt" because what you should be typing is "I have a question".
definition of doubt: <a target="_new" href="http://www.dict.org/bin/Dict?Form=Dict2&Database=*&Query=doubt">click here</a>
<em><b> Doubt \Doubt\ (dout), v. i. [imp. & p. p. Doubted; p. pr. &
vb. n. Doubting.] [OE. duten, douten, OF. duter, doter,
douter, F. douter, fr. L. dubitare; akin to dubius doubtful.
See Dubious.]
1. To waver in opinion or judgment; to be in uncertainty as
to belief respecting anything; to hesitate in belief; to
be undecided as to the truth of the negative or the
affirmative proposition; to b e undetermined.
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Even in matters divine, concerning some things, we
may lawfully doubt, and suspend our judgment.
--Hooker.
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To try your love and make you doubt of mine.
--Dryden.
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2. To suspect; to fear; to be apprehensive. [Obs.]

Syn: To waver; vacillate; fluctuate; hesitate; demur;
scruple; question.
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definition of question: <a target="_new" href="http://www.dict.org/bin/Dict?Form=Dict2&Database=*&Query=question">click here</a>
<em><b>Question \Ques"tion\, n. [F., fr. L. quaestio, fr. quaerere,
quaesitum, to seek for, ask, inquire. See Quest, n.]
1. The act of asking; interrogation; inquiry; as, to examine
by question and answer.
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2. Discussion; debate; hence, objection; dispute; doubt; as,
the story is true beyond question; he obeyed without
question.
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There arose a question between some of John's
disciples and the Jews about purifying. -- John iii.
25.
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It is to be to question, whether it be lawful for
Christian princes to make an invasive war simply for
the propagation of the faith. -- Bacon.
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3. Examination with reference to a decisive result;
investigation; specifically, a judicial or official
investigation; also, examination under torture.
--Blackstone.
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He that was in question for the robbery. Shak.
The Scottish privy council had power to put state
prisoners to the question. --Macaulay.
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4. That which is asked; inquiry; interrogatory; query.
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But this question asked
Puts me in doubt. Lives there who loves his pain ?
--Milton.
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5. Hence, a subject of investigation, examination, or debate;
theme of inquiry; matter to be inquired into; as, a
delicate or doubtful question.
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6. Talk; conversation; speech; speech. [Obs.] --Shak.
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In question, in debate; in the course of examination or
discussion; as, the matter or point in question.

Leading question. See under Leading.

Out of question, unquestionably. "Out of question, 't is
Maria's hand." --Shak.

Out of the question. See under Out.

Past question, beyond question; certainly; undoubtedly;
unquestionably.

Previous question, a question put to a parliamentary
assembly upon the motion of a member, in order to
ascertain whether it is the will of the body to vote at
once, without further debate, on the subject under
consideration.

Note: The form of the question is: "Shall the main question
be now put?" If the vote is in the affirmative, the
matter before the body must be voted upon as it then
stands, without further general debate or the
submission of new amendments. In the House of
Representatives of the United States, and generally in
America, a negative decision operates to keep the
business before the body as if the motion had not been
made; but in the English Parliament, it operates to
postpone consideration for the day, and until the
subject may be again introduced. In American practice,
the object of the motion is to hasten action, and it is
made by a friend of the measure. In English practice,
the object is to get rid of the subject for the time
being, and the motion is made with a purpose of voting
against it. --Cushing.

To beg the question. See under Beg.

To the question, to the point in dispute; to the real
matter under debate.
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Syn: Point; topic; subject.
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you may doubt that there is an answer to your question, although if you wish to find out if there is an answer, you ask a question.
doubt is a state of disbelief (not a state of ignorance), question is a way of finding answers (ignorance seeking knowledge).