Question

write the LCD for each pair4/7, 1/8, 11/12

Answer 1

So if by LCD you mean Least Common Denominator, I would start off with finding values of 7 8 12 that are the same, so by multiplying 7 and 8 you get 56 which is not a multiple of 12.
so we must now find the LCM of 56 and 12 which is 168 found by looking at the progression of numbers 56..112...168 and 12..24..36..48..60..72..and so on.

so to sum up LCM is 168 for all three numbers.56 for 7 and 8. 72 for 12 and 8. or 84 for 7 and 12.

Answer 2

7 = 1x7
8 = 2x2x2
12 = 2x2x3

Everybody must be represented, so
LCD = 1x2x2x2x3x7 = 168