C2:radians measure and its application

(a) Using cosine rule

cosA = 102 + 122 - 1422 × 10 × 12 = 0.2
A = cos-1(0.2) (use in radian mode)
A = 1.369 … = 1.37
----------------------------------------------
i dont get the "use in radian mode"
because cos-1 * 0.2 = 78.5 degree (0.44 rad)

The calculator does a correct job. Cos-1(0.2) = 1.369 (Rad mode)
If you do degrees, Cos-1(0.2) = 78.46 degrees
To convert from degrees to radians, (78.46)*(pi/180) = 1.369 radians which is correct.

However the first line of what you wrote does not make sense.
How does 102 + 122 - 1422 x 10 x 12 = 0.2??