I will try to give a research-related answer. There are several suggestions from the literature and you may have to take a deeper look at them.
First, a decrease in motivation is also observed in other countries, not just Brazil (Frenzel, Goetz, Pekrun, & Watt, 2010).
In recent psychological theories (Krapp, 2002), (Krapp, 2005), (Renninger, 2009), interest development is based on the satisfaction of psychological needs, especially the needs for perceived competence, autonomy and social relatedness from self-determination theory (Deci & Ryan, 1985), (Deci & Ryan, 2002). Students should have these needs satisfied in your teaching as often as possible. But how should this work?
Empirical studies on mathematics interest confirm factors like competence support (Rakoczy, Harks, Klieme, Blum, & Hochweber, 2013) and autonomy support (Valås & Søvik, 1994), but also work on different conceptual levels, e.g. teaching methods (Bikner-Ahsbahs & Halverscheid, 2014), (Lerkkanen u. a., 2012), (Prendergast & O’Donoghue, 2014), (Rowan-Kenyon, Swan, & Creager, 2012), (Schukajlow u. a., 2011), (Sonnert, Sadler, Sadler, & Bressoud, 2015), classroom management (Rowan-Kenyon u. a., 2012), task design (Schukajlow & Krug, 2014) or the big-fish-little-pond effect (Frenzel u. a., 2010), which all more or less explicitly relate to need satisfaction (see also (Carmichael, Callingham, Watson, & Hay, 2009)).
Note, however, that the motivation role of applications tends to be over-estimated by teachers since students may also have interest in pure aspects of mathematics (Rellensmann & Schukajlow, 2016).
The best framework for the design of classroom activities that support the psychological needs and foster students’ motivation is the one by Bikner-Ahsbahs and Halverscheid (2014). In brief, they plan student activities based on exploration and discovery, group work and valuing of (nearly) any student observation and comment. These activities cannot be found for each session (e.g., not for the fostering of calculation routines) but may really help for deep connections of the mathematical content and most of all they may substantially improve students’ motivation.
I am sorry I cannot give a short and simple answer. If we had such an answer, it would have been spread around the world already. Anyway I hope my post may help you.
References:
- Bikner-Ahsbahs, A., & Halverscheid, S. (2014). Introduction to the
Theory of Interest-Dense Situations (IDS). In A. Bikner-Ahsbahs & S.
Prediger (Eds.), Networking of Theories as a Research Practice in
Mathematics Education (pp. 97–113). Cham: Springer International
Publishing. Retrieved from
http://link.springer.com/10.1007/978-3-319-05389-9_7
- Carmichael, C., Callingham, R., Watson, J., & Hay, I. (2009). Factors influencing the development of middle school students’ interest in statistical literacy. SERJ - Statistics Education Research Journal, 8(1), 62–81.
- Deci, E. L., & Ryan, R. M. (1985). Intrinsic Motivation and
Self-Determination in Human Behavior. New York: Plenum Press.
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E. L., & Ryan, R. M. (2002). Handbook of Self-Determination Research:
Theoretical and Applied Issues. Rochester, NY: University of
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