JEE 2023 Mathematics PYQ — Let and be positive real numbers such that the function is differ… | Mathem Solvex | Mathem Solvex
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JEE 2023 — Mathematics PYQ
JEE | Mathematics | 2023
Let k and m be positive real numbers such that the function f(x)={3x2+kx+1,mx2+k2,amp;0amp;x≥1lt;xlt;1is differentiable for all x > 0. Then f′(81)8f′(8) is equal to ________.
Choose the correct answer:
A.
309
(Correct Answer)
B.
308
C.
903
D.
None
Correct Answer:
309
Explanation
f(x)={3x2+kx+1,mx2+k2,amp;0amp;x≥1lt;xlt;1
At x=1 (Continuity): f(1−)=f(1+)⇒3+k2=m+k2…(i)
At x=1 (Differentiability): f′(1−)=f′(1+)⇒6(1)+21+1k=2m(1)
⇒6+22k=2m⇒m=3+42k…(ii)
Substitute (ii) in (i): 3+k2=3+42k+k2
⇒k2+k[421−2]=0
k[k+421−8]=0⟹k=0,427
for k=427,m=3+42427
=3+327=3296+7=32103
So, f′(81)8f′(8)=6×81+427×29188×[2×32103×8]
=43+127412=129+7412=16412×12=309
Explanation
f(x)={3x2+kx+1,mx2+k2,amp;0amp;x≥1lt;xlt;1
At x=1 (Continuity): f(1−)=f(1+)⇒3+k2=m+k2…(i)
At x=1 (Differentiability): f′(1−)=f′(1+)⇒6(1)+21+1k=2m(1)
⇒6+22k=2m⇒m=3+42k…(ii)
Substitute (ii) in (i): 3+k2=3+42k+k2
⇒k2+k[421−2]=0
k[k+421−8]=0⟹k=0,427
for k=427,m=3+42427
=3+327=3296+7=32103
So, f′(81)8f′(8)=6×81+427×29188×[2×32103×8]
=43+127412=129+7412=16412×12=309
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