Y = 8x^8 + 1/4*x^6 + 20x^3 - 4x^2
y ' = 8*8x^7 + 6/4*x^5 + 60x^2 - 4*2x = 64x^7 + 1,5x^5 + 60x^2 - 8x
y '' = 64*7x^6 + 1,5*5x^4 + 60*2x - 8 = 448x^6 + 7,5x^4 + 120x - 8
y''' = 448*6x^5 + 7,5*4x^3 + 120*1 - 0 = 2688x^5 + 30x^3 + 120
y'''' = 2688*5x^4 + 30*3x^2 + 0 = 13440x^4 + 90x^2 = 30x^2 (448x^2 + 3)