# After NumberForm I cannot apply a Sine function. Why?

After NumberForm I cannot apply a Sine function. Why?

This works:

sol1 = x /. Solve[x^2 - 3 == 0, x]
Sin[sol1]


Output = $\{-Sin[\sqrt{3}],Sin[\sqrt{3}]\}$

Also this works:

sol2 = x /. Solve[x^2 - 3 == 0, x] // N
Sin[sol2]


Output = $\{-0.987027,0.987027\}$

Then why doesn't this work? Meaning, why is Sin[] not applied to the elements of the list?

sol3 = NumberForm[x /. Solve[x^2 - 3 == 0, x] // N, 6]
Sin[sol3]


Output = $Sin[\{-1.73205,1.73205\}]$

What can I do to make this work?

• Point nr. 8 here is relevant. – C. E. Sep 3 '15 at 21:42

Use FullForm to see the differences

sol3 = NumberForm[x /. Solve[x^2-3 == 0, x]//N, 6]
(* {-1.73205,1.73205} *)


This definition includes the NumberForm wrapper that was intended just for printing.

sol3//FullForm
(* NumberForm[List[-1.7320508075688772,1.7320508075688772],6] *)


Alternatively, use Information to look at the stored definition

?sol3
(* Globalsol3
sol3=NumberForm[{-1.73205,1.73205},6] *)


Consequently, the input to Sin is neither numeric nor a List of numeric values

Sin[sol3]//FullForm
(* Sin[NumberForm[List[-1.7320508075688772,1.7320508075688772],6]] *)


You could Map the Sin to the appropriate level

Map[Sin,sol3,{2}]//FullForm
(* NumberForm[List[-0.9870266449903538,0.9870266449903538],6] *)


However, the common approach is to put the wrapper outside the definition

NumberForm[sol32=x/.Solve[x^2-3==0,x]//N,6]//FullForm
(* NumberForm[List[-1.7320508075688772,1.7320508075688772],6] *)


However, you have to use NumberForm again if you want the output of Sin to also be formatted with NumberForm

Sin[sol32]//FullForm
(* List[-0.9870266449903538,0.9870266449903538] *)

NumberForm[Sin[sol32],6]//FullForm
(* NumberForm[List[-0.9870266449903538,0.9870266449903538],6] *)

NumberForm[N[x /. Solve[x^2 - 3 == 0, x]], 6]

Sin[%]

(* {-1.73205,1.73205} *)

(* {-0.987027, 0.987027}*)

• The first input needs to be evaluated before you apply Sine function, therefore use Sin[%] – thils Sep 3 '15 at 10:58
• Hmm, interesting. But why isn't the Sin applied directly when I do Sin[sol3]? – GambitSquared Sep 3 '15 at 10:58
• Yeah, what is going on here? This doesn't answer the question of "Why?" at all. Taking Sin[sol3] doesn't get evaluated. But typing sol3;Sin[%] does. So what gives? – Jason B. Sep 3 '15 at 11:01
• @thils On my computer (MMA 10.2 Win7-64) running your code outputs Sin[{-1.73205,1.73205}]`, not the values you show. How did you generate this behavior? – MarcoB Sep 3 '15 at 16:09
• @MarcoB pls delete ";" , have edited this bit. Numberform attributes are described in the manual. – thils Sep 3 '15 at 23:18