They were ____ ____ ____ ____a word when they met again 20 years later.
用数学归纳法证明不等式 1+12+13+K+1n<2nn∈N*
老张、老王、老李、老赵是好朋友,一天,他们四人碰面,每两人都握一次手,他们一共握了多少次手?(先画一画,再解答.)