Так, момент. Покажите весь конфиг урлов и ссылку, как она у вас прописана в шаблоне.
urlpatterns = [
url(r'^$', views.index),
url(r'^hello/$', views.hello),
url(r'^shapka/$', shapka),
url(r'^time/$', views.current_date_time),
url(r'^matem/$', matem),
url(r'^admin/', include(admin.site.urls)),
url(r'^resume/$', resume),
url(r'^left/$', left),
url(r'^right/$', main_post),
url(r'^center/$', center),
url(r'^develop/$', views.develop),
url(r'^develop/matem/$', views.matem),
]
<a href="matem"> Треугольник </a></br>
