How can I simplify numerical calculations in symbolic math toolbox?

Hi there,

When I write this:

fR == simplify((Alpha/uR)*((y/uR)^(Alpha-1))*exp(-(uS/y)^k))

I get something like this:

fR =

(12153*exp(-(10/y)^(601/100))*(y/20)^(11153/1000))/20000

Could you tell me please how to simplify the numerical calculations?! I mean I want the term "(11153/1000))/20000" be printed as "0.000557".

Thanks

Answer 1

[Star Strider]

Use the vpa function. (If you want to solve an equation and avoid fractions, use the vpasolve function.)

[Antonio]

Thanks for your response. It makes the results better but doesn't completely solve the issue. I get the following when I use vpa:

fR =

0.60765*exp(-1.0*(10.0/y)^(601/100))*(0.05*y)^(11153/1000)

So, if this is limited to (11153/1000) that would be fine of course! But do you know if other calculations are always done when you use vpa? Thanks

[Star Strider]

Actually, that’s essentially what you want. It’s just expressing the decimal fractions literally instead of as decimals. However, I’m not quite certain why it’s rendering, for instance, (11153/1000) as a fraction and not as 11.153. Maybe close enough is good enough for vpa. (I get the same result as you do.)

[Walter Roberson]

Exponents are not converted to floating point because a floating point exponent means something different than a rational exponent. With rational exponents you need to worry about multiple roots and principle roots, but with floating point exponent the operation x^(a/b) is defined strictly in terms of exp(a/b * log(x)) which is a single value that is complex in cases where algebraic x^(a/b) might be real-valued.