原式=(1+1/2)-(3-1/6)+(3+1/12)-(5-1/20)+...+(9+1/90)
=(1-3+3-5+5-7+7-9+9)+(1/2+1/6+1/20+...+1/90)
=1+(1-1/2+1/2-1/3+...+1/9-1/10)
=1+(1-1/10)
=1.9