Let $z=f(x,y)=20x^2-16xy+25y^2.$ Find the following using the formal definition of the partial derivative.

a$.\frac{\text{d}\text{e}\text{l}\text{z}}{\text{d}\text{e}\text{l}\text{x}}$

b$.\frac{\text{d}\text{e}\text{l}\text{z}}{\text{d}\text{e}\text{l}\text{y}}$

c$.\frac{\text{d}\text{e}\text{l}\text{f}}{\text{d}\text{e}\text{l}\text{x}}(1,4)$

d$.f_y(1,-2)$