$($$\dfrac{1}{3.5}$$.x$$-3)+($$\dfrac{1}{5.7}$$.x-5)+($$\dfrac{1}{7.9}$$.x-7)+...+($$\dfrac{1}{19.21}$$.x-19)=-99$$\dfrac{2}{35}$
$⇔$$\dfrac{x}{3}$$-$$\dfrac{x}{5}$$-3+$$\dfrac{x}{5}$$-$$\dfrac{x}{7}$$-5+$$\dfrac{x}{7}$$-$$\dfrac{x}{9}$$-7+...+$$\dfrac{x}{19}$$-$$\dfrac{x}{21}$$-19=-99$$\dfrac{2}{35}$
$⇔$$\dfrac{x}{3}$$-$$\dfrac{x}{5}$$+$$\dfrac{x}{5}$$-$$\dfrac{x}{7}$$+$$\dfrac{x}{7}$$-$$\dfrac{x}{9}$$+...+$$\dfrac{x}{19}$$-$$\dfrac{x}{21}$$-3-5-7-...-19=-99$ $\dfrac{2}{35}$
$⇔$$\dfrac{x}{3}$$-$$\dfrac{x}{21}$$-99=-99+$$\dfrac{2}{35}$
$⇔$$\dfrac{7x}{21}$ $-$ $\dfrac{x}{21}$$=$$\dfrac{2}{35}$
$⇔$$\dfrac{6x}{21}$$=$$\dfrac{2}{35}$
$⇔$$x=$$\dfrac{21.2}{6.35}$$=$ $\dfrac{1}{5}$
$\text{Mệt bở hơi tai}$
