50 баллов
Найдите ac , если :
a=((1^2)+(2^2)+(3^2)+.........+(16^2) - 16)/(1*3+2*4+3*5+........15*17)  и 
c=(1-(1/2))(1-(1/3))(1-(1/4))(1-(1/5))(1+(1/5))(1+(1/4))(1+(1/3))(1+(1/2)).