MATLAB: How to simplify numerical calculations in symbolic math toolbox

simplifysymbolic math

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

Best Answer

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

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When I test in a different software package, I find no problem with the representation, even if I convert the floating point numbers to rationals. I do not find any portion of the expression to be particularly large.

An algebraic equivalent to the expression is

(16191151046901145333338734592/77084264605915775) * I4^(3705992092344839/1125899906842624) - (2558136220007576158378628793351/9487294105343480) * I5 * I4^(2580092185502215/1125899906842624) + (3335311470258831134378888053767/9487294105343480) * I5 * I4^(3705992092344839/562949953421312)

Caution: remember that all of those fractions are going to be evaluated to floating point numbers, losing precision in the process. If you do not want that, then you need to use the Symbolic Toolkit, with code such as

Cov__B_C_direct = sym('1125899906842624') * exp(I4^(sym('4831891999187463') / sym('1125899906842624'))) * exp(sym('8187485477687969') / sym('1715238139330560')) * ((sym('230090095199403') * exp(-I4^(sym('4831891999187463') / sym('1125899906842624'))) * exp(-sym('8187485477687969') / sym('1715238139330560')) * I4^(sym('3705992092344839') / sym('1125899906842624'))) / sym('1233348233694652400') - (sym('2558136220007576158378628793351') * exp(-I4^(sym('4831891999187463') / sym('1125899906842624'))) * exp(-sym('8187485477687969') / sym('1715238139330560')) * I5 * I4^(sym('2580092185502215') / sym('1125899906842624'))) / sym('10681743549394799937933824491520') + (sym('3335311470258831134378888053767') * exp(-I4^(sym('4831891999187463') / sym('1125899906842624'))) * exp(-sym('8187485477687969') / sym('1715238139330560')) * I5 * I4^(sym('3705992092344839') / sym('562949953421312'))) / sym('10681743549394799937933824491520'))

Or in the reduced form,

Cov__B_C_direct = (sym('16191151046901145333338734592') / sym('77084264605915775')) * I4^(sym('3705992092344839') / sym('1125899906842624')) - (sym('2558136220007576158378628793351') / sym('9487294105343480')) * I5 * I4^(sym('2580092185502215') / sym('1125899906842624')) + (sym('3335311470258831134378888053767') / sym('9487294105343480')) * I5 * I4^(sym('3705992092344839') / sym('562949953421312'))

MATLAB: Converting fractions to decimals in exponents of symbolic expressions

maybe like this:

syms x
   a=sym('3.7')
   f=sym((2*x.^(a)))
   digits(20)
   a=vpa(diff(f,x))

Best Answer

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