求教数学题目

waterjuice

    In a land far, far away all the lily pads grow in a straight line across the rivers and are always the same distance apart. Frogs when they jump from lily pad to lily pad can either go to the adjacent lily pad of jump over the adjacent lily pad to the next. Kermit the frog wishes to cross a river which has 3 lily pads. How many ways can Kermit cross a river which has

    8 lily pads                                                               

    20 lily pads


用什么技巧，