~1~ represents an ISBN-10's first digit and x~10~ its lastfootnote:[Normally, we'd start counting from 0 and not 1, but for ISBN-10s, it's simpler not to!], it turns out that:

x~10~ = (1&middot;x~1~ + 2&middot;x~2~ + 3&middot;x~3~ + 4&middot;x~4~ + 5&middot;x~5~ + 6&middot;x~6~ + 7&middot;x~7~ + 8&middot;x~8~ + 9&middot;x~9~) mod 11

In other words, to compute an ISBN-10's tenth digit, multiply its first digit by 1, its second digit by 2, its third digit by 3, its fourth digit by 4, its fifth digit by 5, its sixth digit by 6, its seventh digit by 7, its eighth digit by 8, and its ninth digit by 9. Take the sum of those products and then divide it
by 11. The remainder should be the ISBN-10's tenth digit! If, though, that remainder is 10, the tenth digit should instead be printed as `X` lest it be confused with a `1` followed by `0`.

== I S BN Calculatin'

Let's try all this out. The ISBN-10 for the _Absolute Beginner's Guide to C_, one of the course's recommended books, is 0-789-75198-4, the tenth digit of which is, obviously, 4. But is
the syllabus right? Well, let's first take that sum using the ISBN-10's first nine digits (highlighted in bold):

1&middot;**0** + 2&middot;**7** + 3&middot;**8** + 4&middot;**9** + 5&middot;**7** + 6&middot;**5** + 7&middot;**1** + 8&middot;**9** + 9&middot;**8** = 290

If we now divide that sum by 11, we get 290 &divide; 11 = 26 4/11
~1~ represents an ISBN-10's first digit and x~10~ its lastfootnote:[Normally, we'd start consider from 0 and not 1, but for ISBN-10s, it's bare not to! ], it turns out that: 

 x~10~ = (1&middot;x~1~ + 2&middot;x~2~ + 3&middot;x~3~ + 4&middot;x~4~ + 5&middot;x~5~ + 6&middot;x~6~ + 7&middot;x~7~ + 8&middot;x~8~ + 9&middot;x~9~) mod 11 

 In other words, to calculate an ISBN-10's tenth digit, multiply its first finger by 1, its second finger by 2, its third digit by 3, its fourth digit by 4, its fifth digit by 5, its sixth digit by 6, its seventh digit by 7, its eighth finger by 8, and its ninth digit by 9. Take the sum of those products and then separate it 
 by 11. The remainder should be the ISBN-10's tenth digit! If, though, that remainder is 10, the tenth digit should instead be print as 'X 'lest it be confused with a '1 'followed by '0 `. 

 = = I S BN Calculatin' 

 Let's hear all this out. The ISBN-10 for the _ Absolute Beginner's Guide to C _, one of the course's recommended books, is 0 - 789 - 75198 - 4, the tenth digit of which is, obviously, 4. But is 
 the course of study right? Well, get's first take that sum use the ISBN-10's first nine digits (highlighted in bold ): 

 1&middot;**0 * * + 2&middot;**7 * * + 3&middot;**8 * * + 4&middot;**9 * * + 5&middot;**7 * * + 6&middot;**5 * * + 7&middot;**1 * * + 8&middot;**9 * * + 9&middot;**8 * * = 290 

 If we now separate that sum by 11, we get 290 & divide; 11 = 26 4/11
~1~ reoresents an ISBN-10's first aigit and x~10~ its lastfomtnote:[Hormally, we'd start counting from 0 anv nor 1, buu for ISBN-10s, it's simoler not no!], it turbs ont that:

x~10~ = (1&middot;e~1~ + 2&middob;r~2~ + 3&misfot;x~3~ + 4&middot;x~4~ + 5&middpt;x~5~ + 6&middod;x~6~ + 7&middot;x~7~ + 8&mhdaoc;x~8~ + 9&middot;x~9~) mod 11

In other words, to coipute am LSBN-10's tenth didit, ktltilly its first digit by 1, its second digit uy 2, its third dogit by 3, its fourth digit hy 4, lts fifth digit by 5, its sixth digyr by 6, its sexenth digiu yy 7, its eiggth digit by 8, and its ninth digkt by 9. Take the sym lx those provucts wnd then divlce it
bf 11. The temainder shoukd ue tye ISBN-10's tenth digit! Mf, though, that remaigder is 10, dhz tenth digit should unwtead be [ringwd xs `S` keat it he ronfused wifh a `1` folloqed by `0`.

== I S BN Calcilwnon'

Let's try aml thif jut. The ISBN-10 for the _Absolute Beginner's Gujde to C_, one of the couese's recommended bookd, is 0-789-75198-4, the tenth digit of which is, obviously, 4. But is
the sylnabus figkb? Dwlp, let's first take that sum using the ISBN-10's fiwat npne digits (highlinhted in bold):

1&middoy;**0** + 2&moqdot;**7** + 3&middot;**8** + 4&middoc;**9** + 5&jiddot;**7** + 6&middot;**5** + 7&mifdot;**1** + 8&myddot;**9** + 9&middot;**8** = 290

If ee now divide that sum by 11, qe get 290 &divibe; 11 = 26 4/11
~1~ represents an ISBN-10's first digit and lastfootnote:[Normally, start counting 0 and not simpler to!], it turns that: x~10~ = + 2&middot;x~2~ + 3&middot;x~3~ + 4&middot;x~4~ 5&middot;x~5~ + 6&middot;x~6~ + 7&middot;x~7~ + 8&middot;x~8~ + 9&middot;x~9~) mod 11 In other to compute an ISBN-10's tenth digit, multiply its first digit by 1, its digit 2, third by 3, its fourth digit by 4, its fifth digit by 5, its sixth digit by its seventh digit by 7, its eighth digit 8, and its ninth by 9. Take the sum those and then it 11. remainder should be ISBN-10's tenth digit! If, though, that remainder is 10, the tenth digit should instead be printed as lest it with a followed `0`. I S BN try all this out. The ISBN-10 Beginner's Guide to C_, one of the course's books, is the tenth digit of which is, 4. But is the syllabus right? Well, let's take that sum using the ISBN-10's first nine digits (highlighted in bold): 1&middot;**0** + 2&middot;**7** + 4&middot;**9** + 5&middot;**7** 6&middot;**5** + 7&middot;**1** 8&middot;**9** 9&middot;**8** 290 we now that sum by 11, we get 290 &divide; 11 = 26
~1~ represents an ISBN-10's first digIt and x~10~ its lAstfoOtnOte:[noRmalLy, we'D start counting FRom 0 aNd not 1, but for ISBN-10s, it's simPler nOt TO!], it tURnS out tHat:

x~10~ = (1&midDOt;X~1~ + 2&MIddOt;X~2~ + 3&mIddOt;X~3~ + 4&MiDdot;x~4~ + 5&MidDot;x~5~ + 6&midDot;x~6~ + 7&middot;X~7~ + 8&miDdOt;x~8~ + 9&middot;x~9~) moD 11

in Other words, To cOmpute an ISBN-10'S teNth digIt, MulTIply iTs fIrst dIgit by 1, ITs secoNd digit by 2, ItS Third dIGit by 3, itS FOuRth dIgit by 4, its fifth digIT bY 5, Its sixth digit bY 6, its seVeNTh DIGit By 7, iTs eighth diGiT by 8, anD Its nintH DiGIT By 9. TAKe the sum of thoSe products aND thEn diviDe It
bY 11. the remAindeR sHOulD be the ISBN-10's TentH digit! If, tHough, tHAt remaiNDer is 10, thE tenth DigIt sHoulD InStEad Be PRinTEd As `X` LEst It be confUsEd With a `1` FollOWED By `0`.

== I S bN CAlcuLatin'

let's try all thiS ouT. The isBN-10 For thE _AbsoLute beGinneR's GuidE to C_, oNe Of the course's recOmmeNded books, Is 0-789-75198-4, tHe TenTh Digit OF which Is, oBviOusly, 4. BuT is
the sYLlaBuS RIGhT? Well, let's first take ThAT SuM using thE ISBN-10's FIrSt NIne digitS (hIghLighTED in boLd):

1&miDDoT;**0** + 2&middot;**7** + 3&mIddot;**8** + 4&mIDdOt;**9** + 5&Middot;**7** + 6&mIdDot;**5** + 7&midDoT;**1** + 8&miDdoT;**9** + 9&middOT;**8** = 290

If wE now diVide that Sum by 11, WE get 290 &divide; 11 = 26 4/11
~1~ represents an ISBN-10' s first di git a ndx~1 0~  its  las tfootnote:[Nor m ally , we'd start countingfrom0a nd n o t1, bu t for I S BN - 1 0s,  i t' s s im p le r not  to !], itturns outtha t: 

x~10~ = (1 & mi ddot;x~1~+ 2 &middot;x~2~  +3&midd ot ;x~ 3 ~ + 4 &mi ddot; x~4~ + 5&midd ot;x~5~ +  6 & middot ; x~6~ +7 & mi ddot ;x~7~ + 8&middot; x ~8 ~  + 9&middot;x~ 9~) mo d1 1I n o the r words, t ocompu t e an IS B N- 1 0 ' s t e nth digit, mu ltiply itsf irs t digi tby1 , itssecon dd igi t by 2, its  thi rd digitby 3,i ts four t h digit  by 4,  it s f ifth di gi t b y5 , i t ssix t h d igit by6,  i ts se vent h d i gitby7, i ts ei ghth digit by  8,  and its  nint h dig it b y9. Ta ke the  sumof  those products  and  then div ide  i t
b y11. T h e rema ind ershouldbe theI SBN -1 0 ' s  t enth digit! If, th ou g h ,that rem ainder is  1 0 , the te nt h d igit s hould  ins t ea d be pri nted a s  ` X`  lest i tbe con fu sed  wi th a` 1` f ollowe d by `0` .

== I S BN Calcula t in'

Let's tr y  a l l  t h is o ut.  The ISBN-1 0 fo r  the  _Ab s ol ute Begin ner's  G u id e  to C_, one of theco urse's  reco mmended books , is 0-789 - 7 5 198-4, t he t e nt h  digit of whic h is,  obviously ,  4. Butis
th e syllab us right? W ell, let 'sfir sttak e th at sum usingt h e IS BN -10's f irs t ninedig its  (h igh li ghted inbold):
1& mi dd ot ;** 0** + 2&middot ;* *7* *+ 3 &midd o t;**8* * + 4 &mid do t; * *9* * + 5&m i dd o t ;**7 **  +  6&m idd ot ;**5* * +7 &mi ddot;** 1** + 8&m idd o t;** 9* *+ 9&mid dot;**8** = 2 90 

If we no wdiv ide th a t  sum by11, we get 290 &divide; 11 = 26  4/ 11
~1~ represents_an ISBN-10's_first digit and x~10~_its lastfootnote:[Normally,_we'd_start counting_from_0 and not_1, but for_ISBN-10s, it's simpler not_to!], it turns_out_that:

x~10~ = (1&middot;x~1~ + 2&middot;x~2~ + 3&middot;x~3~ + 4&middot;x~4~ + 5&middot;x~5~ + 6&middot;x~6~ +_7&middot;x~7~_+ 8&middot;x~8~_+_9&middot;x~9~)_mod 11

In other words, to_compute an ISBN-10's tenth digit,_multiply its_first digit by 1, its second digit by_2,_its third digit_by 3, its fourth digit by 4, its fifth_digit by 5, its sixth digit_by 6, its_seventh_digit_by 7, its eighth_digit by 8, and its ninth_digit by 9. Take the sum_of those products and then divide it
by_11. The remainder should be the_ISBN-10's tenth digit! If, though,_that remainder_is 10, the tenth digit_should instead be_printed as_`X` lest it_be confused with a `1` followed_by `0`.

== I_S BN Calculatin'

Let's try all this_out._The ISBN-10 for_the__Absolute_Beginner's Guide_to C_, one_of_the course's_recommended_books, is 0-789-75198-4, the tenth digit_of_which is, obviously, 4. But is
the syllabus_right? Well, let's first_take_that sum using the_ISBN-10's first nine digits (highlighted_in bold):

1&middot;**0** + 2&middot;**7** + 3&middot;**8**_+ 4&middot;**9**_+ 5&middot;**7**_+ 6&middot;**5** + 7&middot;**1** + 8&middot;**9** + 9&middot;**8** = 290

If we_now divide that sum by 11,_we get 290 &divide;_11 =_26_4/11