>>461
t/2=sとおくと
(f(x,y-t/2)-f(x,y+t/2))/t
=(f(x,y-s)-f(x,y)+f(x,y)-f(x,y+s))/2s
=(1/2)*(f(x,y-s)-f(x,y))/s+(1/2)*(f(x,y)-f(x,y+s))/s
lim[s→0]をとると以下略