Posted by **please..anyone help!!** on Monday, March 18, 2013 at 3:11am.

How many integers appear in both of the following arithmetic progressions:

A1: 2, 9, 16,.....2+(1000-1)*7

A2: 3, 12, 21,.....3+(1000-1)*9

Since 2 appears in A1 but not in A2, it does not appear in both of the arithmetic progressions.

- number theoRY!!! -
**Anonymous**, Monday, March 18, 2013 at 5:25am
A1_n= 2+(n-1)*7=7n-5

A2_k= 3+(k-1)*9=9k-6

Put A1_n=A2_k => so 9k-7n=1 and solve the Diophantine equation for n,k u can see that gcd(9,7)=1 so the solutions you want come from applying the euclidean algorithm and your numbers n,k must be less than or equal to 1000.

- dio -
**Im**, Monday, March 18, 2013 at 5:52am
Calculating GCD(9,7) gives:

7 = 0*9 + 7

9 = 1*7 + 2

7 = 3*2 + 1

2 = 2*1 + 0

Then applying the Extended Euclidean Algorithm:

1 = (1 * 7) + (-3 * 2)

= (-3 * 9) + (4 * 7)

= (4 * 7) + (-3 * 9)

A particular solution is:

x0 = -3

y0 = 4

The complete solution is:

x = -3 + 7n

y = 4 - 9n

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