Q3 / 118 · NTA Abhyas (JEE Main) 2020 · Mathematics · Sequences and Series

Let l, m and n are three distinct numbers in arithmetic progression. Also l2, m2 and n2 are in geometric progression and l+m+n=3. If l<m<n, then n is equal to

  • A
    1
  • B
    1-2
  • Correct answer
    C
    1+2
  • D
    2+5

Solution

l,m,n are in A.P. 

⇒2m=l+n

Given, l+m+n=3⇒m=1

Let, l=1-d, n=1+d, d>0
Now, m4=l2n2⇒m2=-ln or ln
So 1+d1-d=±1

⇒d=0 or 2 (d=0 is rejected)
Hence, n=1+2

Hence, the correct answer is option: 'C'.

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