OCR Core 2 Jan 2008 part ii

I am writing up some solutions.
This is a Cos rule Question. We agree on part (i)


For part ii I get 0.4036 as the Cos of the Angle. The mark scheme gets 0.3998.

Here is my working $[br]\cos \text{BAD}=\frac{20^2+10^2-18.4^2}{2\times20\times 10}=0.4036[br]$

Here is their's
Section A $18.4^2 =10^2 +20^2 –2 \times10 \times 20 \times \cos \theta >>>> cos \theta = 0.3998$

Where am I going wrong? Marks schemes are usually right.

No, it's right if you use the exact value, which is 18.441699581062914766900917908956. This then gives cos theta =
0.3997592914045597849455132519797