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Problems in Mathematical Analysis.pdf - pwp.net.ipl.pt

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80 Differentiation of Functions [Ch. 2<br />

Example 2. Compute<br />

limf-J- L^ (form oo<br />

*-M)V s<strong>in</strong> x x J<br />

Reduc<strong>in</strong>g to a common denom<strong>in</strong>ator, we get<br />

oo).<br />

lim (4__J_Ulim*.!z^lf (fonn<br />

x-+o \ s<strong>in</strong> 2 x x 2<br />

J x-o x z s<strong>in</strong> 2 x<br />

v '<br />

Before apply<strong>in</strong>g the L'Hospital rule, we replace the denom<strong>in</strong>ator of the latter<br />

fraction by an equivalent <strong>in</strong>f<strong>in</strong>itesimal (Ch. 1, Sec. 4) * 2 s<strong>in</strong> 2 A;~ x*. We<br />

obta<strong>in</strong><br />

The L'Hospital rule gives<br />

lim ( L) = lim<br />

Then, <strong>in</strong> elementary fashion, we f<strong>in</strong>d<br />

_ ___<br />

x- \ s<strong>in</strong> 2 x x 2<br />

Example 3. Compute<br />

lim (cos 2x) * 2<br />

X-M)<br />

A "~ S<br />

m *=lim-<br />

a<br />

1 cos 2x ,. 2 s<strong>in</strong> 2 * 1<br />

8<br />

(form I<br />

00<br />

)<br />

Tak<strong>in</strong>g logarithms and apply<strong>in</strong>g the L'Hospital rule, we get<br />

lim In (cos 2*p = lim 31ncos2* = _ 6 lim = _ 6 .<br />

X-*0 X-+Q X x-+Q %X<br />

J^<br />

Hence, lim (cos 2x) x *^e-*.<br />

Jf->0<br />

ples.<br />

F :<br />

<strong>in</strong>d the <strong>in</strong>dicated limits of functions <strong>in</strong> the follow<strong>in</strong>g exam-<br />

77G. lim x'-<br />

,_>,<br />

Solution. lim<br />

*'<br />

777. lim xcosx<br />

11IU tjm<br />

I-*<br />

"<br />

; ^r2 X->1 OJi 7 "~~ /<br />

vr s<strong>in</strong>x , 779. lir<br />

jix '<br />

*->il_s<strong>in</strong> :r 780. lim<br />

~~ 9 *<br />

*7on is tan* s<strong>in</strong>*<br />

o

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