Let 2n (equally spaced) points on a circle be chosen. show that the number of ways to join these points in pairs, so that the resulting n line segments do not intersect, equals the nth catalan number cn.

International calls require the use of a country code. many country codes a 3-digit numbers. country codes do not begin with a 0 or 1. there are no restrictions on the second and third digits. how many different 3-digit country codes are possible?

Answer asap evaluate 4-0.25g+0.5h4−0.25g+0.5h when g=10g=10 and h=5h=5.

2,10,-50 next two numbers geometric sequence