1.2/3+2/9+2/27+2/81+2/243
=2/3+(1/3-1/9)+(1/9-1/27)+(1/27-1/81)+(1/81-1/243)
=2/3+1/3-1/243
=242/243
2.设儿子x岁,则父亲（x+28）岁
根据题意 12/（x+28）=5/12
解得 x =20
所以儿子是20岁