1)sn *(1-q)= (1 - q ^ n),
sn = (1 - q ^ n)/(1 - q)而lim q ^ n = 0(n -> 无穷)所以lim sn = 1/(1 - q)
2)y=x(x-1)(x+1)^(1/2)/(x^3-1)
=x(x+1)^(1/2)/(x^2+x+1)=(x+1)^(1/2)/(x+1+1/x)
只考虑x,y是实数,分子x 的次数低于
分母x的次数,所以当x趋于
无穷大时y~1/(x+1)^(1/2)是
无穷小。
x-->0, y~1/(1+1/x)是无穷小
x>-1,x-->-1,y~(x+1)^(1/2)/(-1)是无穷小
3)
极限lim(x趋近无穷大)x^(1/2)((x+2)^(1/2)-2(x+1)^(1/2)+x^(1/2))
=极限lim(x趋近无穷大)(x^2+2x)^(1/2)-2(x^2+x)^(1/2)+x
~极限lim(x趋近无穷大)(x^2)^(1/2)-2(x^2)^(1/2)+x=0