In Lesson 26, we learned Vてくる ("begin to V") expression referring to uncontrollable (natural) events. Vていく ("V-ing is realized (over time) ") is another such compound verb form referring to uncontrollable (natural) events. This form expresses a change of state experienced over time as "going away from" the current state of affairs or a continued activity resulting in gradual build up or melt away of some value in the future. (In contrast, Vてくる expresses a change of state experienced as "coming closer to" the current state of affairs.)

	Vて	いく
だんだん (gradually)	ふとって (gain weight) わかって (understand)	いく (go)	gradually gain weight gradually understand
暖かく	なって (become)		become warmer
もっともっと (more and more)	べんきょうして		keep on studying more and more
少しずつ (little by little)	わすれて		forget little by little

	ダイエットしなかったら、ふとっていくわ。 ダイエットしなかったから、ふとってきたわ。	If I don't diet, I will gradually gain weight. Because I didn't diet, I have gained weight.
	コンピュータは安くなっていきますよ。 コンピュータは安くなってきましたよ。	Computers will become cheaper. Computers have become cheaper.
	コンピュータはべんりになっていきますよ。 コンピュータはべんりになってきましたよ。	Computers will become more convenient. Computers have become more convenient.

Note that the particle に is required after a noun or な-adjective: べんりになっていく.