interactive mode 
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An interactive mode suggests that users could have different choices. 
  For example, they are given a menu for 
    1) pv_f
    2) pv_perpetuity
    3) pv_perpetuity_due
    4) pv_annuity
    5) pv_annuity_due
    6) fv_f
    7) fv_annuity
    8) pv_annuity_due
   -9) exit

 A typical example. 
   > .searchWebs("fincal")
       NAME                           WEBSITE
    1 fincal http://datayyy.com/exe/fincal.exe
Key components: 
    readline() fucntion 
    cat(' Enter your PV 
')
    if(){
    }else if{

    }else if{

    } else{
  
   }
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pv(annuity) derivation
////////////////////////////////
Logic: A annuity can be treated as two perpetuities. 
Assume that we receive $10 for the next 5 years. It can be treated as :
  1) we receive $10 for forever
  2) starting from year 6, we also pay $10 forever

Why? The reason is that the formula of a perpetuity is so simple
           c       c       c        c 
   |-------|-------|-------| ...    |
           1       2       3        n
1)
           c       c       c        c      c   -> infinity
   |-------|-------|-------| ...    |------|
           1       2       3        n     n+1  -> infinity

2)                                         -c  -> infinity
   |-------|-------|-------| ...    |------|  
           1       2       3        n      n+1 -> infinity

pv(annuity) = pv(perpetuity #1) - pv(perpetuity #2)
            = c/R   - c/R *1/(1+R)^n
            = c/R (1  - 1/(1+R)^n)
////////////////////////////////

A simple x, y graph (x as horizontal axis while y is for vertical axis)
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Assume that we have the following two columns. 
         where y= 3+3x
x       y
1       6
2       9
3       12
4       15
5       18
6       21
7       24
8       27
9       30
10      33
11      36

  Step 1: highlight both columns
  Step 2: Click "Insert" -> Line -> All chart types
  Step 3: choose "X Y (scatter"
  Step 4: click " OK"

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