题目

  I was in my first year of college, making friends and enjoying life, but then my whole world turned upside down. I had a heart attack. It felt like someone was stabbing (刺) me in the chest with a knife over and over again.    After three months of rest I went back to college, but then things took a turn for the worse. I was staying at my Nana’s house and woke up in the middle of the night with a terrible pain in my chest. I knew I was having a heart attack again.    I couldn’t even shout for help. Luckily my grandpa was going to the toilet and heard me falling out of bed.已知n条直线l1：x-y+C1=0，C1=，l2：x-y+C2=0，l3：x-y+C3=0，…，ln：x-y+Cn=0（其中C1＜C2＜C3＜…＜Cn），这n条平行直线中，每相邻两条直线之间的距离顺次为2、3、4、…、n．（1）求Cn；（2）求x-y+Cn=0与x轴、y轴围成的图形的面积；（3）求x-y+Cn-1=0与x-y+Cn=0及x轴、y轴围成图形的面积．