Ayuda por favor necesito explicacion de como se hace

Find the total area for the regular pyramid ( follow the answer set up in the picture)

Question 5 spring semester final exam math evaluate

For the rule of 78, for a 12-month period, the last term in the sequence is 12 and the series sums to 78. for an 10 month period, the last term is and the series sum is . for a 15 month period, the last term is and the series sum is . for a 20 month period, the last term is and the series sum is