1.y'=7x^6+6x^5-15x^4
2.y'=1-x^-2
3.y'=6x(x-5)+3x^2+2
4.y'=15x^2(3x+8)+3(5x^3-7)
5.y'=(x^2+1-2x^2)/(x^2+1)^2
6.y'=(xcosx-sinx)/x^2