10 POINTS NEED HELP ASAP

Find the value of the cosine for angle a

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

Find an bif a = {3, 6, 9, 12) and b = {2, 4, 6, 8, 10).