1) f(x)=sinx+cosx , f'(x)=cosx-sinx , f'(0)=cos0-sin0=1-0=1
2) y=1/4*x⁴-2/3*x³+1/2*x²-x+2
y'=x³-2x²+x-1 , y'(-1)= -1-2-1-1= -5
3) y=(x²+1)(x³-1) , y'=2x(x³-1)+3x²(x²+1)=2x⁴-2x+3x⁴+3x²=5x⁴+3x²-2x
y'(1)=5+3-2=6
