Lim x->0 V7+x-V7-x/5x
lim ( V7+x-V7-x)(V7+x + V7-x) /5x*(V7+x + V7-x)=lim 7+x-(7-x) /5x*(V7+x +V7-x)=lim 2x/5x*(V7+x+V7-x)=lim2/5*(V7+x+V7-x)=2/5*2V7=1/5V7