The value of 4co$t^2$45° − se$c^2$60° + si$n^2$30° + co$s^2$90° is:

cot45° = 1 makes the first term 4; sec60° = 2 makes the second 4; sin30° = 1/2 makes the third 1/4; and cos90° = 0 makes the last 0. The total is 4 − 4 + 1/4 + 0 = 1/4. Taking cos90° = 1 pushes the answer to 5/4, confusing sec60° with cos60° leaves 4 − 1/4 + 1/4 = 4, and stopping after the two big terms gives 0.