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Formula
Find the sum or difference of the fractions
$\left( - \dfrac{ 2 }{ 7 } \right) + \dfrac{ 4 }{ 21 }$
$- \dfrac { 2 } { 21 }$
Find the sum or difference of the fractions
$- \dfrac { 2 } { \color{#FF6800}{ 7 } } + \dfrac { 4 } { \color{#FF6800}{ 21 } }$
 The smallest common multiple in denominator is $21$
$- \dfrac { 2 } { \color{#FF6800}{ 7 } } + \dfrac { 4 } { \color{#FF6800}{ 21 } }$
$- \dfrac { 2 } { 7 } + \dfrac { 4 } { 21 }$
 Multiply the denominator and the numerator so that the denominator is the smallest common multiple 
$- \dfrac { 2 \times \color{#FF6800}{ 3 } } { 7 \times \color{#FF6800}{ 3 } } + \dfrac { 4 } { 21 }$
$\color{#FF6800}{ - } \color{#FF6800}{ \dfrac { 2 \times 3 } { 7 \times 3 } } \color{#FF6800}{ + } \color{#FF6800}{ \dfrac { 4 } { 21 } }$
 Organize the expression 
$\color{#FF6800}{ - } \color{#FF6800}{ \dfrac { 6 } { 21 } } \color{#FF6800}{ + } \color{#FF6800}{ \dfrac { 4 } { 21 } }$
$\color{#FF6800}{ - } \color{#FF6800}{ \dfrac { 6 } { 21 } } \color{#FF6800}{ + } \color{#FF6800}{ \dfrac { 4 } { 21 } }$
 Since the denominator is the same as $21$ , combine the fractions into one 
$\color{#FF6800}{ \dfrac { - 6 + 4 } { 21 } }$
$\dfrac { \color{#FF6800}{ - } \color{#FF6800}{ 6 } \color{#FF6800}{ + } \color{#FF6800}{ 4 } } { 21 }$
 Add $- 6$ and $4$
$\dfrac { \color{#FF6800}{ - } \color{#FF6800}{ 2 } } { 21 }$
$\dfrac { \color{#FF6800}{ - } \color{#FF6800}{ 2 } } { 21 }$
 Move the minus sign to the front of the fraction 
$\color{#FF6800}{ - } \color{#FF6800}{ \dfrac { 2 } { 21 } }$
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