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Question Number 205092 by BaliramKumar last updated on 08/Mar/24

Answered by A5T last updated on 08/Mar/24

n^2 +19n+92−x^2 =0  n=((−19+_− (√(361−4(92−x^2 ))))/2)=((−19+_− (√(4x^2 −7)))/2)  4x^2 −7=p^2 ⇒(2x−p)(2x+p)=7=1×7=−1×−7  2x−p=1∧2x+p=7⇒x=2  2x−p=−1∧2x+p=−7⇒x=−2  n=((−19+_− 3)/2)=−8 or −11⇒sum=−19⇒(d)

n2+19n+92x2=0n=19+3614(92x2)2=19+4x2724x27=p2(2xp)(2x+p)=7=1×7=1×72xp=12x+p=7x=22xp=12x+p=7x=2n=19+32=8or11sum=19(d)

Commented by BaliramKumar last updated on 08/Mar/24

Thanks sir

Thankssir

Answered by A5T last updated on 08/Mar/24

(n+9)^2 <n^2 +19n+92<(n+10)^2   when n≥0, so n^2 +19n+92 can′t be a perfect square  (n+10)^2 =n^2 +20n+100=n^2 +19n+92+n+8  when n=−8, then n^2 +19n+92=(−8+10)^2 =4  but when n<−8, (n+10)^2 <n^2 +19n+92  (n+9)^2 =n^2 +18n+81=n^2 +19n+92−n−11  when n=−11,n^2 +19n+92=(−11+9)^2 =4  but when n<−11; (n+9)^2 >n^2 +19n+92  ⇒when n<−11; (n+10)^2 <n^2 +19n+92<(n+9)^2   Equality holds at either −8 or −11  So,one can easily check n=−10,−9,−7,−6,...−1  to see that none satisfies.

(n+9)2<n2+19n+92<(n+10)2whenn0,son2+19n+92cantbeaperfectsquare(n+10)2=n2+20n+100=n2+19n+92+n+8whenn=8,thenn2+19n+92=(8+10)2=4butwhenn<8,(n+10)2<n2+19n+92(n+9)2=n2+18n+81=n2+19n+92n11whenn=11,n2+19n+92=(11+9)2=4butwhenn<11;(n+9)2>n2+19n+92whenn<11;(n+10)2<n2+19n+92<(n+9)2Equalityholdsateither8or11So,onecaneasilycheckn=10,9,7,6,...1toseethatnonesatisfies.

Commented by BaliramKumar last updated on 08/Mar/24

difficult method  example      n^2  + 2n + 361        twelve value of   n

difficultmethodexamplen2+2n+361twelvevalueofn

Commented by A5T last updated on 08/Mar/24

This sort of method could be useful for questions  like n^4 +n^3 +n^2 +n+1=x^2 (Q204397) where it   would be difficult to make n the subject of   formula in terms of x.  So,methods have their disadvantages and  advantages.

Thissortofmethodcouldbeusefulforquestionsliken4+n3+n2+n+1=x2(Q204397)whereitwouldbedifficulttomakenthesubjectofformulaintermsofx.So,methodshavetheirdisadvantagesandadvantages.

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