Calculus (3rd Edition)

Published by W. H. Freeman
ISBN 10: 1464125260
ISBN 13: 978-1-46412-526-3

Chapter 14 - Calculus of Vector-Valued Functions - 14.2 Calculus of Vector-Valued Functions - Exercises - Page 720: 42

Answer

$\mathop \smallint \limits_0^1 \left( {t{{\rm{e}}^{ - {t^2}}}{\bf{i}} + t\ln \left( {{t^2} + 1} \right){\bf{j}}} \right){\rm{d}}t = \left( { - \frac{1}{2}\left( {{{\rm{e}}^{ - 1}} - 1} \right)} \right){\bf{i}} + \left( {\ln 2 - \frac{1}{2}} \right){\bf{j}}$

Work Step by Step

Evaluate $\mathop \smallint \limits_0^1 \left( {t{{\rm{e}}^{ - {t^2}}}{\bf{i}} + t\ln \left( {{t^2} + 1} \right){\bf{j}}} \right){\rm{d}}t$. Since vector-valued integrals obey the same linearity rules as scalar-valued integrals, so $\mathop \smallint \limits_0^1 \left( {t{{\rm{e}}^{ - {t^2}}}{\bf{i}} + t\ln \left( {{t^2} + 1} \right){\bf{j}}} \right){\rm{d}}t = \left( {\mathop \smallint \limits_0^1 t{{\rm{e}}^{ - {t^2}}}{\rm{d}}t} \right){\bf{i}}$ ${\ \ \ \ }$ $ + \left( {\mathop \smallint \limits_0^1 t\ln \left( {{t^2} + 1} \right){\rm{d}}t} \right){\bf{j}}$ Evaluate $\mathop \smallint \limits_0^1 t{{\rm{e}}^{ - {t^2}}}{\rm{d}}t$ Write $v = {t^2}$. So, $dv = 2tdt$. The integral becomes $\mathop \smallint \limits_0^1 t{{\rm{e}}^{ - {t^2}}}{\rm{d}}t = \frac{1}{2}\cdot\mathop \smallint \limits_0^1 {{\rm{e}}^{ - v}}{\rm{d}}v = - \frac{1}{2}{{\rm{e}}^{ - v}}|_0^1$ $\mathop \smallint \limits_0^1 t{{\rm{e}}^{ - {t^2}}}{\rm{d}}t = - \frac{1}{2}\left( {{{\rm{e}}^{ - 1}} - 1} \right)$ Evaluate $\mathop \smallint \limits_0^1 t\ln \left( {{t^2} + 1} \right){\rm{d}}t$ Write $v = {t^2} + 1$. So, $dv = 2tdt$. The integral becomes $\mathop \smallint \limits_0^1 t\ln \left( {{t^2} + 1} \right){\rm{d}}t = \frac{1}{2}\mathop \smallint \limits_1^2 \ln v{\rm{d}}v$ Recall from integration by parts formula: $\smallint u{\rm{d}}v = uv - \smallint v{\rm{d}}u$ Write $u = \ln v$. So, $du = \frac{1}{v}dv$. The integral becomes $\frac{1}{2}\mathop \smallint \limits_1^2 \ln v{\rm{d}}v = \frac{1}{2}\left[ {\left( {v\ln v} \right)|_1^2 - \mathop \smallint \limits_1^2 v\cdot\frac{1}{v}{\rm{d}}v} \right]$ $\frac{1}{2}\mathop \smallint \limits_1^2 \ln v{\rm{d}}v = \frac{1}{2}\left[ {\left( {v\ln v} \right)|_1^2 - v|_1^2} \right]$ $\frac{1}{2}\mathop \smallint \limits_1^2 \ln v{\rm{d}}v = \frac{1}{2}\left[ {2\ln 2 - \ln 1 - 1} \right] = \frac{1}{2}\left( {2\ln 2 - 1} \right)$ $\frac{1}{2}\mathop \smallint \limits_1^2 \ln v{\rm{d}}v = \ln 2 - \frac{1}{2}$ From the results above we obtain $\mathop \smallint \limits_0^1 \left( {t{{\rm{e}}^{ - {t^2}}}{\bf{i}} + t\ln \left( {{t^2} + 1} \right){\bf{j}}} \right){\rm{d}}t = \left( { - \frac{1}{2}\left( {{{\rm{e}}^{ - 1}} - 1} \right)} \right){\bf{i}} + \left( {\ln 2 - \frac{1}{2}} \right){\bf{j}}$
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