Evaluate the following limits or explain why they don't exist. y²– 2xy (a) lim (x,y) (1,-2) y + 3x 4xy (b) lim (x,y) (0,0) 3x² + y² (c) lim (d) lim (x,y) →(-1,1) 2x² - xy - 3y² x + y y²- - x² (x,y) →(0,0) y4 + 2x²