Question 1196364
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First things first:  When you want to indicate a subscript in plain text, use an underscore, thus:  a_3 means *[tex \Large a_3].  If there is more than one character in the subscript, then enclose the subscript in curly braces, thus:  a_{10} means *[tex \Large a_{10}].  You don't say so (and you most certainly should have) but I will assume an arithmetic sequence.


From *[tex \Large a_{10}] to *[tex \Large a_{12}] is two terms of the sequence.  The difference between the two terms given is *[tex \Large a_{12}\ -\ a_{10}\ =\  -45\ -\ (-37)\ =\ -8]. Since that is the difference between terms 10 and 12, the common difference for the sequence must be half of that, or *[tex \Large -4].  The number of terms from *[tex \Large a_{10}] to *[tex \Large a_1] is *[tex \Large 10\ -\ 1\ =\ 9].  Then *[tex \Large 9\ *\ -4\ =\ -36]


So:  *[tex \Large a_1\ =\ -37\ -\ (-36)\ =\ -1]

														
John
*[tex \LARGE e^{i\pi}\ +\ 1\ =\ 0]
My calculator said it, I believe it, that settles it
*[illustration darwinfish.jpg]

From <https://www.algebra.com/cgi-bin/upload-illustration.mpl> 
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