A line OA of length r starts from its initial position OX and traces an angle AOB=alpha in the anticlockwise direction.It then traces back in the clockwise direction an angle BOC=3 theta (where alpha is greater than 3 theta).L is the foot of the perpendicular form C on OA . I want to understand what this means. Will this question be on topic here?
The OP's question doesn't look to be on topic for this site. The excerpt is too long. Questions about meaning is usually reserved for single words, expressions or short phrases, see the results here to get a better idea.
The excerpt is relevant to mathematics, right? No?
Searching the keywords alpha, theta, angle on Mathematics.SE site reveals that is probably the most suitable site. Their help page doesn't explicitly forbid questions that ask about meaning; in fact it says questions that ask about understanding is on topic.
- Understanding mathematical concepts and theorems.
The same help page offers alternative sites that the OP may find fits the bill.
In any case, in the OP's shoes, I would first ask mathematics.meta before posting this type of question.
I am not 100% sure on this and I would like to see what the other members think, but in an advisory capacity I would say probably not. Our scope specifically addresses word choice and usage, etymology, syntax, dialectical differences, pronunciation, spelling and punctuation. Asking us to interpret an entire paragraph exceeds those bounds by a considerable margin. I do not even think we would answer a question regarding how to interpret an entire sentence, except maybe for syntactical purposes: Just specific portions of a sentence. We might ask for more than that in some circumstances to get a sense of the general context though.
Speaking of excess, there are limits to what a Stack Exchange question may address, and if you can not restrict the scope of the question to a reasonable extent, then the question can be closed as too broad irrespective of where you ask it on the S.E. network. The guidelines for what constitutes too broad are somewhat vague though:
Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question.
The most confusing thing about this paragraph seems to be the abbreviations, and the question may be closed as unclear if they turn out to be context sensitive abbreviations which are indecipherable out of context.
Finally, it does not seem like something within our field of expertise. Granted, in some cases we may address the terminology of other arts, but our website is for etymologists, linguists and serious English language enthusiasts, whereas what you are asking is something that sounds like something that needs the help of a geometry expert to fully understand. Now perhaps we might have a geometry expert, but if we do then their existence here is more or less incidental. There is a geometry tag at Mathematics S.E. though, and their scope includes a number of related S.E. subdomains you might consider for more specific subject matter. Granted, I am not a member of their community, so I am even less sure about what exactly is acceptable there, but it seems like a better bet there than here.
Now if you are having trouble interpreting a specific portion of those sentences, we may or may not be able to help you with that, depending on the portion that is giving you trouble.
As an aside, it should also be noted that there are certain guidelines for quoting material written by others. Every post utilizing quoted material should include the quotation to be delimited with either quotation marks or markdown, and cited with both the name of the author and the title of the source. Would you please inform us of that information, or at least let us know if your teacher wrote that as part of a homework question or something?