Figure 1 - Discretized Square Plate Exercises and Explorations (A) [20] Write a system of (four by four) linear equations for the node values x1,x2,x3, and x4 and clearly identify the matrix A in the matrix equation Ax=b. Do this by averaging the values at each node x1,x2,x3, and x4 by taking the (thermal) average of its (four) adjacent nodes (with the given boundary conditions). [That is, write an equation for each internal node where the thermal value for that node is the average of the four adjacent nodes.]