lim(-1+3^n)/2^(n+1)=lim[(-1/3^n)+1/[(2/3)^n*2]当n趋向正无穷时3^n趋向正无穷所以-1/3^n趋向负无穷小而(2/3)^n趋向正无穷小所以分子是1+负无穷小分母是正无穷小所以lim(-1+3^n)/2^(n+1)[n趋向正无穷] 趋向正无穷
极限为无穷大上下同除以3^n
