Find limx→0+parctan(√x/p)−qarctan(√x/q)x√x
without L'Hopital rule and series expansions.
Could someone help me , I did not understand how to find it, thanks.
Answer
Recall arctany=∫y01/(1+t2)dt. Verify that
1−t2≤11+t2≤1−t2+t4
for all t. Thus for y>0,
y−y3/3≤arctany≤y−y3/3+y5/5.
It follow that our expression is bounded below by
p(√x/p−x3/2/(3p3))−q(√x/q−x3/2/(3q3)+x5/2/(5q5))x3/2.
Simplify to see this →1/(3q2)−1/(3p2). There is a similar estimate from above, giving the same limit. By the squeeze theorem, the limit is 1/(3q2)−1/(3p2).
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