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Different result command window and guide

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john
john on 6 Mar 2014
Edited: Mischa Kim on 7 Mar 2014
Hello,
if I write in command window:
t=1
4.5582758767110180218764941687633*sin(314.0*t) - 16.557811525543529374688574837543*cos(314.0*t) + (16.557811525543529374688574837543*( 3.4028e+308*(1375.8856146096578468897575988832*t) + 0.48398685287825251446260421986529* 3.4028e+308*(1375.8856146096578468897575988832*t)))/exp(1485.7142857142857142857142857143*t)
I get NaN,but in guide I get -17.0711.....why?

Answers (1)

Mischa Kim
Mischa Kim on 6 Mar 2014
John, in your expression you have two huge terms, basically,
Inf/Inf
which results in a NaN. You should get the same result for the same expression in your GUI.
  3 Comments
Mischa Kim
Mischa Kim on 7 Mar 2014
You can do
syms t
expr = 92.9*cos(314.0*t) + 96.3*sin(314.0*t) - (7.94*(cosh(1399.0*t) + 4.06*sinh(1399.0*t)))/exp(1500.0*t);
expr = subs(expr, sinh(1399.0*t),(exp(1399.0*t) - exp(-1399.0*t))/2);
expr = subs(expr, cosh(1399.0*t),(exp(1399.0*t) + exp(-1399.0*t))/2);
expr = expand(expr,'ArithmeticOnly',true);
ezplot(expr,[0,0.1])
The two subs titutions are hyperbolic trig identities, thereby avoiding the Inf/Inf issue (in combination with the expand command).

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