Is this correct?

[ptex]

an = 2n2 - 4n + 7
n>1
-
4
Eai 5+7+13+9=34
i=1

a4 = 2(4)2 - 4(4) + 7 = 32 - 23 = 9
34?

[pig]

Check your a4.. 7 is added, not substracted.

[ptex]

How about this
an = 2n2 - 4n + 7
n>1
-
4
Eai 5+7+13+23=48
i=1

a4 = 2(4)2 - 4(4) + 7 = 32 - 16 + 7 = 23
48?

[ptex]

How about this one;
Y={2,5} C={1,2}
List the elements of [C], the equivalence class containg C
{1}{2}{1,5}{1,2,5}

The example in my book is the same question but Y={3,4} and C={1,3}
and the solution is {1}{1,3}{1,4}{1,3,4}

[ptex]

Is 48 correct for the first one?