# Hi All. Just wondering why answer to (i) is not 4.9e+4. an why answer to (iii) is not (9x)/2 or 4.5 x. Many thanks. Brian

3 visualizzazioni (ultimi 30 giorni)
Brian Smyth il 5 Lug 2024
Commentato: Voss il 5 Lug 2024
% (i) Find f(1.2) in the form 'a x 10^n' where a is correct to 1 decimal place
% (ii) Find x for which g(x) = 3.5
% (iii) Find g(f(x)) in simplest form
clear, clc
syms x
f = exp(9*x); g = log(sqrt(x));
% (i)
f_at_x = subs(f,x,1.2); % evaluates f(x = 1.2) as symbolic
f_at_x = double(f_at_x); % converts to double (real)
f_at_x = round(f_at_x,1) % ??? round to 1 decimal place
f_at_x = 4.9021e+04
% (ii)
solve (g == 3.5, x)
ans =
% (iii)
g_f_x = (subs(g,x,f));
g_f_x = simplify(g_f_x)
g_f_x =
##### 0 CommentiMostra -2 commenti meno recentiNascondi -2 commenti meno recenti

Accedi per commentare.

### Risposte (2)

Aquatris il 5 Lug 2024
Your value in (i) for f(1.2) is 49020.80113638172, so when you round to 1 decimal, it becomes 49020.8 which in engineering notation is 4.9021e4 because of rounding the right most digit.
You should reread the question to understand why you are not getting 4.9 as answer (hint:question is asking a where f(1.2) = a * 10^n ) :)
syms x
f = exp(9*x); g = log(sqrt(x));
% (i)
f_at_x = subs(f,x,1.2); % evaluates f(x = 1.2) as symbolic
format long % change displasy setting
f_at_x = double(f_at_x) % converts to double (real)
f_at_x =
4.902080113638172e+04
f_at_x_rounded = round(f_at_x,1) % ??? round to 1 decimal place
f_at_x_rounded =
4.902080000000000e+04
format bank % change displasy setting
f_at_x_rounded
f_at_x_rounded =
49020.80
For part ii, log(x^n) = n*log(x) so both expressions are equivalent
fun1 = @(x) 0.5*log(exp(9*x));
fun2 = @(x) log(exp(4.5*x));
fun1(1.2) == fun2(1.2) % are they equal?
ans = logical
1
##### 7 CommentiMostra 5 commenti meno recentiNascondi 5 commenti meno recenti
Brian Smyth il 5 Lug 2024
Spostato: Torsten il 5 Lug 2024
Hi Voss.
That's good to know. Never quite understood the difference between the two (i..e fprintf and sprintf)! Thank you.
Voss il 5 Lug 2024
You're welcome!

Accedi per commentare.

Torsten il 5 Lug 2024
Spostato: Voss il 5 Lug 2024
For (iii), use
syms x real
syms x
##### 1 CommentoMostra -1 commenti meno recentiNascondi -1 commenti meno recenti
Brian Smyth il 5 Lug 2024
Spostato: Voss il 5 Lug 2024
Hi Torsten. Brilliant. Thanks a mil. Brian

Accedi per commentare.

### Categorie

Scopri di più su Symbolic Variables, Expressions, Functions, and Preferences in Help Center e File Exchange

### Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by