Weird answer while using Log

syms n

x = some value

why do i get weird answer when i do n*log10(x)?

for ex: if i do 10*log10(8)i get output as 9.03. But when i include n*10*log10(8) i get

 (20335778644494715*n)/2251799813685248

as the output

Answer 1

[Star Strider]

Different variable classes:

syms n

Q1 = 10*log10(8)
ClassQ1 = class(Q1)

Q2 = n*10*log10(8)
ClassQ2 = class(Q2)

Q1 =
     9.0309e+000
ClassQ1 =
double

Q2 =
(20335778644494715*n)/2251799813685248
ClassQ2 =
sym

If you want to change the appearance of the output, use the vpa function:

Q2 = vpa(Q2, 5)

Q2 =
9.0309*n

Answer 2

[Walter Roberson]

Look at http://www.mathworks.com/help/symbolic/sym.html#input_argument_flag and notice that the flag 'r', "rational" is the default. That means that all floating point numbers in MATLAB are automatically converted to rational form by default when they become associated with a symbolic expression. You can force a different interpretation by using sym() around the number with the appropriate conversion flag, such as

n*10*sym(log10(8),'d')