Error message on code which runs

[BillFoster]

See advice in MAS1041CBA3_5 and in particular the advice for the second question. The cursor on the eqnarray there reports an error although the question compiles and runs. I inserted simplify and displaystyle in various places and this appears to be the reason why the error message appeared in the first place(?)

[BillFoster]

The problem appear to be with copying and pasting; for example I copied \displaystyle{ and pasted it in several locations adding the appropriate right brace each time. The error then is reported (although there is none) or there is no display of the maths when you put the cursor on that piece of latex,

[BillFoster]

This is because I mistakenly put braces in the \displaystyle environment for some stupid reason. After removing these all is OK.