For the first, the product is and for the second, it's telescopic.
well, I know that i must develop product, so when i do it, then cancel whats possible in nominator and denominator, and lets say i had left some for etc (n+1)/(n+2), what shoud i do with cos (n!+100)/n^2 ?, or in the 1st problem, when i develop product, and cancel whats possible, what to do with sin(...) ?