Grammar Section " Will and Be Going To"

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this is conversation among radit and Kenzo as follows:
Radit   : The weekend is arriving,what are you going to do?
Kenzo : My brother and I are going to enjoy cave tubing at Pindul Cave....
Radit   : Pindul cave? where is it?
Kenzo : It's inGunung Kidul, Yogyakarta. what about you?
Radit   : I'll do sports or watch movies. I have no plan yet.
Kenzo : What if you join us?
Radit   : Hmmmmm,,,, Let me think first. I'll call you later.
Kenzo : All right.

Both of the boys are talking about their activities at the coming weekend. Kenzo and his brother are going to enjoy cave tubing at pindul cave, while Radit doesn't have a plant yet. Perhaps, he'll do sports or watch a movie.
The words be going to is used to express a prediction.

1. Either will or be going to is used express a prediction.
    e.g.
    * According to the weather report, it will rain tomorrow.
       According to the weather report, it is going to rain tomorrow.

    * She will travel to Bali next holiday.
    * She is going to travel to Bali next holiday.

2. be going to is used to talk about something which the speaker intends to do in the future because she/he
    has made a plan to do it.
    e.g.
    * Enggar   : Are you free this afternoon?
    * Anni       : I don't think so. Sally and I are going to find some reference books for our sociology
                       assignments at Grand bookstore.
    * Intan      : How long are you going to stay in Surabaya?
    * Husni     : We're going to stay for a week.

3. Will is used to express Willingness
     e.g.
     * Rorik      : It is hot here
        Diandra  : I'll turn the AC on

     * Lizzy  : Let's have a surprisr party for Dad.
        Elisie  : Sounds like a good idea.
        Lizzy  : O.K> I'll decorate the room
        Elisie  : And I'll Prepare the cake