I need some help with sequence and figuring out an equation for given sequence.

For example,

what is a equation for a sequence 10,6,10,6,...

another one is -1/4,-2/9,-3/16/-4/25

for this one I thought it would be -(n+1)/(n+2)^2 But i got it wrong.

third one is a1 = 10, a(n+1) = an/(an+1). how would u find a2, a3, a4, a5 from the give information?

Thanks.