Determine which of the differentials (or both) is the total differential: a) 2[cos(2x + 2y) + x] dx - [4y sin(2x + 2y) + 1] dy; [10 marks] b) 6 sin(y) e+dx +(3(siny + cosy)ex+y-2 y sin(y)dy. +y [20 marks] Find the potential function U(x, y) subject to the condition U(0, 0) = 5 in the case of the total differential. Rubric: 4 marks if the derivative with respect to x was determined correctly in the case (a); 4 marks if the derivative with respect to y was determined correctly in the case (a); 2 marks if the conclusion regarding the total differential was correct in the case (a); 4 marks if the derivative with respect to x was determined correctly in the case (b); 4 marks if the derivative with respect to y was determined correctly in the case (b); 2 marks if the conclusion regarding the total differential was correct in the case (b); 6 marks if the potential function was derived correctly in case (b); 2 marks if the constant of integration was found correctly in case (b); 2 marks if the potential function was presented explicitly in the final form in case (b).