A problem by edil tizon
Level
2
The sequence a,1,b is a non-constant arithmetic progression. The sequence 1,a,b is a geometric progression. Find b.
The answer is 4.
This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try
refreshing the page, (b) enabling javascript if it is disabled on your browser and,
finally, (c)
loading the
non-javascript version of this page
. We're sorry about the hassle.
1 solution
Since a,1,b is an arithmetic progression, a+b=2, therefore b=2−a. By the geometric progression\'s property, we get a2=b=2−a a2+a−2=0: a1=1 or a2=−2. But the arithmetic progression is non-constant, therefore a≠1, so a must be -2. b=2−a=2−(−2)=4.