4.6.5 · D1 · Maths › Ordinary Differential Equations › Bernoulli equations — substitution
Ek Bernoulli equation ek aasaan ("linear") equation ki tarah almost hoti hai, bas ek term ki wajah se bigdi hoti hai jahan y kisi ajeeb power par hoti hai. Poora method yeh hai: y ke ek hisse ko ek naye letter se rename karo taaki woh annoying power gayab ho jaaye , aur ek friendly equation bacha rahe jo tumhe pehle se solve karni aati hai.
Is page par assume kiya gaya hai ki tumne is notation mein se kuch bhi nahi dekha. Hum har ek symbol ko bilkul shuruaat se banate hain, uss order mein jis order mein unhe ek doosre ki zaroorat hai, phir tumhe parent Bernoulli topic par wapas bhejte hain.
Kisi bhi equation se pehle, actors par agree karo.
Definition Unknown function
y aur input x
x ek plain number hai jo hum freely choose kar sakte hain — socho iske baare mein "horizontal axis par kitni door hain" hum.
y koi fixed number nahi hai. Yeh ek rule hai jo har x ke liye ek height deta hai . Hum y ( x ) likhte hain matlab "jab input x ho tab height kya hai."
Picture: ek curve kaagaz par bani hui. Apni ungali x ki kisi value par neeche wali axis par slide karo; curve tumhe y batata hai, wahan ki height.
Topic ko yeh kyun chahiye: ek differential equation ek ऐसी curve ke baare mein ek statement hai jo hum abhi nahi jaante . Neeche sab kuch uss curve ki talash hai.
d x d y = curve ki slope
Curve par ek point par itna zoom karo jab tak woh ek seedhi line jaisi na lage. Uski steepness — x ke ek step mein kitni height y milti hai — derivative kehlata hai, likha jaata hai d x d y ya y ′ .
Upar jaana → d x d y > 0 .
Flat → d x d y = 0 .
Neeche jaana → d x d y < 0 .
Do naam kyun? d x d y yaad dilata hai ki kya change ho raha hai (y mein ek choti change aur x mein ek choti change ka ratio); y ′ sirf shorthand hai jab meaning clear ho. Yeh dono ek hi cheez hain .
Topic ko yeh kyun chahiye: ek Bernoulli equation mein d x d y hoti hai . Yeh humein har point par slope batati hai — uss position ke hisaab se — ek recipe jo curve ko maanni padti hai.
Definition Ordinary Differential Equation (ODE)
Ek aisi equation jo ek curve ki height y ko uski slope d x d y (aur input x ) se joдти hai. "Ordinary" = sirf ek input variable x . "First-order" = sabse zyada slope-taking hum sirf ek baar karte hain (koi y ′′ nahi).
Intuition ODE ko ek "slope field" ki tarah padho
d x d y = (kuch involving x , y ) jaisi ek ODE tumhe plane ke har point par ek chota arrow deti hai jo batata hai ki uss point se guzarne wali curve kis taraf jhukni chahiye. Solve karna = ek aisi curve khinchna jo hamesha arrows follow kare.
Bernoulli mein jo bhi khaas hai woh sab y ki power mein hai. Toh powers theek se samjho.
Topic ko specifically y 1 − n kyun chahiye: substitution v = y 1 − n naam deta hai. 1 − n aata kahan se hai? Arithmetic dekho: equation se y n ki power cancel karne ke liye hum usse divide karte hain, y ko y 1 − n = y 1 / y n mein badal deta hai (division rule use karke upar se). Woh ek subtraction 1 − n hi poore trick ki pehchaan hai.
1 − n padhna
n = 2 ⇒ 1 − n = − 1 ⇒ v = y − 1 = y 1 .
n = 2 1 ⇒ 1 − n = 2 1 ⇒ v = y .
n = 4 ⇒ 1 − n = − 3 ⇒ v = y − 3 .
P ( x ) aur Q ( x ) — coefficients jo x ke saath badal sakte hain
Yeh sirf numbers hain jo is baat par depend karte hain ki tum kahan ho , y par nahi. d x d y + P ( x ) y = Q ( x ) y n mein:
P ( x ) plain y term ko scale karta hai (e.g. P = x 1 ).
Q ( x ) mushkil y n term ko scale karta hai (e.g. Q = x ).
Picture: aisi dials jinki settings x slide karne par change hoti hain, lekin jo y kabhi nahi dekhti .
Topic ko yeh distinction kyun chahiye: hum equation ko rescue kar paate hain kyunki P aur Q mein y nahi hoti. Saari y ki mushkil uss akele y n mein packed hai. Usse isolate karo, khatam karo, kaam ho gaya.
Definition Linear first-order ODE
Ek ODE y mein linear hoti hai jab y aur d x d y sirf first power par aate hain, kabhi saath multiply nahi hote, kabhi roots ya squares mein nahi hote:
d x d y + P ( x ) y = Q ( x ) .
Koi y 2 nahi, koi y nahi, koi y ⋅ y ′ nahi.
Intuition "Linear" kyun promised land hai
Linear equations ke liye ek guaranteed recipe hai (integrating factor — dekho Linear First-Order ODEs — Integrating Factor ) jo unhe hamesha crack kar deta hai. Bernoulli mein y n woh ek cheez hai jo linearity todti hai. Use hata do aur tum ghar ho.
Common mistake "Linear" ka matlab
seedhi line nahi hai
Kyun sahi lagta hai: "linear" word se lines yaad aati hain. Trap: yahan yeh describe karta hai ki y equation mein kaise aata hai (sirf first power), solution curve ki shape nahi — jo wildly curvy ho sakti hai.
Yeh sabse important tool hai, toh hum ise dhyan se banate hain.
Agar v y se bana hai, aur y apni taraf se x ke saath change hota hai, toh x ke respect mein v ki slope hai
d x d v = y par v ka reaction d y d v × x par y ka reaction d x d y .
Ise ek relay ki tarah padho: x mein ek nudge y ko nudge karta hai, jo v ko nudge karta hai. Dono "nudge rates" multiply karo.
wahi choice kyun hai
Woh result dekho: isme exactly y − n d x d y hai — bilkul wohi lump jo parent topic y n se divide karke banata hai. Toh v ′ uss mushkil term ko poora nigal leta hai , aur jo bachta hai usme koi buri power nahi hoti. Exponent 1 − n reverse-engineer kiya gaya tha taaki d y d v y − n nikale. Isliye koi aur substitution nahi.
Ise poora tum Linear First-Order ODEs — Integrating Factor mein miloge; yahan bas symbols decode karo taaki kuch bhi black box na rahe.
μ ( x ) ke pieces
∫ ⋯ d x = antiderivative : ek derivative ko undo karo, matlab woh function dhoondo jiska slope andar wali cheez hai. Geometrically, ek curve ke neeche accumulated area.
e ( ) = exponential function, woh curve jo apni khud ki height ke barabar rate se grow karti hai . Yeh isliye aata hai kyunki yeh woh ek function hai jo slopes ke ek product ko ek clean derivative ( μv ) ′ mein "undo" karta hai, integrate karne ke liye ready.
Toh μ ( x ) = e ∫ ( 1 − n ) P d x ek chalaki se chuna gaya multiplier hai jo linear ODE ke left side ko ek single product ke derivative mein badal deta hai, integrate karne ke liye ready.
Topic ko yeh kyun chahiye: jab Bernoulli ko linear form v ′ + ( 1 − n ) P v = ( 1 − n ) Q mein flatten kar diya jaata hai, yeh μ woh crank hai jo tum finish karne ke liye ghumate ho.
C — family label
Integration hamesha ek unknown constant C chhodti hai, kyunki kaafi curves ek hi slope field share karte hain, sirf vertical shift ya scaling mein differ karte hain. C woh naam ka tag hai jo ek curve ko infinite family mein se choose karta hai . Ek extra fact (ek initial condition jaise y ( 1 ) = 2 ) C ko ek number par pin karta hai.
derivative dy/dx as slope
ODE links height and slope
divide by y^n to expose y^1-n
P of x and Q of x coefficients
Khud ko test karo — har line ka answer chhupa hua hai.
d x d y kya measure karta hai, ek word mein?Curve ka slope (steepness) ek point par.
y − 2 aur y 1/2 ko exponents ke bina likhna.y 0 kya hai, aur yeh n = 1 case kyun break karta hai?y 0 = 1 ; n = 1 ke saath, v = y 1 − 1 = y 0 = 1 constant hai — useless.
Kya P ( x ) aur Q ( x ) kabhi y par depend karte hain? Nahi — yeh sirf x par depend karte hain.
d x d v ke liye chain rule batao jab v y par depend kare aur y x par.d x d v = d y d v ⋅ d x d y .
v = y 1 − n ko x ke respect mein differentiate karo.d x d v = ( 1 − n ) y − n d x d y .
Ek ODE "y mein linear" kab hoti hai? y aur y ′ sirf first power par aate hain; koi y n nahi, y aur y ′ ke koi products nahi.
∫ ⋯ d x kya karta hai?Antiderivative dhoondhta hai — woh function jiska slope integrand hai (area accumulation).
Integrate karne ke baad arbitrary constant C kyun aata hai? Kaafi curves ek slope field share karte hain; C choose karta hai kaun sa.