If the arithmetic mean of two numbers is 10 and geometric mean is 6, then the numbers are
If the arithmetic mean of two numbers is 10 and geometric mean is 6, then the numbers are
A) 1, 19
B) 2, 18
C) 3, 17
D) 3, 18
2 Answers
Correct option is (B) 2, 18
Let both required numbers are a and b.
\(\therefore\) Their arithmetic mean \(=\frac{a+b}2\)
and their geometric mean \(=(ab)^\frac12\)
\(\therefore\frac{a+b}2=10\;\&\;(ab)^\frac12=6\) (Given)
\(\Rightarrow a+b=20\) _______________(1)
& \(ab=6^2=36\) _______________(2)
Now, \((a-b)^2=(a+b)^2-4ab\)
\(=20^2-4\times36\)
\(=400-144\)
\(=256=16^2\)
\(\therefore a-b=16\) _______________(3)
By adding (1) & (3), we get
\((a+b)+(a-b)=20+16\)
\(\Rightarrow2a=36\)
\(\Rightarrow a=\frac{36}2=18\)
\(\therefore b=20-a\)
\(=20-18=2\) (From (1))
Hence, both numbers are 2 and 18.