what is the LCD of 2/7 and 9/10

The least common denominator or LCD is 70 because for

10=10,20,30,40,50,60,70,80

7=7,14,21,28,35,42,49,56,63,70,77

and 70 is the least common denominator of 10 and 7 so 70 ims your answer

Assuming LCD means Lowest Common Denominator

Assuming by "and" you mean adding the two

Well, you go by the multiples of 7;

7, 14, 21, 28, 35, 42, 49, 56, 63, 70

Multiples of 10;

10, 20, 30, 40, 50, 60, 70, 80, 90, 100

Straight away you can see the lowest common multiple is 70,

Now to go about adding them,

2/7 * 10 (both sides), 20/70

9/10 * 7 (both sides), 63/70

Then you add the two; 20/70 + 63/70 - now we can add them because they share the same denominator

20/70 + 63/70 = 83/70 = 1 whole 13/70 (simplified)

The LCD is 70

Quick tip, to find a common multiple (not necessarily the lowest) multiply the two denominators.