for first one take 5n cmmon , then (4/5)n →0 as n→∞ so limit is 5
second is similar , take xn common , y/x bcomes fraction, and (y/x)n tends to zero as n →∞
for third , rationalise first, then put y = -x , and then check wat shud be done to keep the limit finite i.e 0 here.
for fourth use the half angle formula of 1- cosx = 2sin2(x/2)
i.e 1-cos(sinx) = 2sin2((sinx)/2)