Помогите 4 задание !!!!!!!!!!!!

Решите задачу:

$b_n=b_1\cdot q^{n-1}\\\\ b_1= \frac{1}{4} \\\\ b_2= \frac{1}{4} \cdot (-\frac{1}{2})^{2-1} = -\frac{1}{4} \cdot \frac{1}{2} = -\frac{1}{8} \\\\ b_3= \frac{1}{4} \cdot (-\frac{1}{2})^{3-1} = \frac{1}{4} \cdot (-\frac{1}{2})^2 = \frac{1}{4}\cdot \frac{1}{4} = \frac{1}{16} \\\\ S_n= \frac{b_1(1-q^n)}{1-q} \\\\$

$S_7= \frac{\frac{1}{4}(1-( -\frac{1}{2} )^7)}{1-(- \frac{1}{2} )} =\frac{\frac{1}{4}(1+\frac{1}{128})}{1+ \frac{1}{2}} =\frac{\frac{1}{4}\cdot \frac{129}{128}}{ \frac{3}{2}} = \frac{129}{512} \cdot \frac{2}{3} = \frac{43}{256}$