1.
1080÷12×(1+3)=360棵
2.
1/2+1/6+1/12+...+1/90
=1/(1×2)+1/(2×3)+1/(3×4)+...+1/(9×10)
=1-1/2+1/2-1/3+1/3-1/4+...+1/9-1/10
=1-1/10
=9/10