Question

Найдите сумму первых трех членов последовательности (an) с общим членом. 60 !

Answer 1

$69)a_{n}=\frac{2-n}{2n}\\\\a_{1} =\frac{2-1}{2*1}=\frac{1}{2}\\\\a_{2}=\frac{2-2}{2*2}=0\\\\a_{3}=\frac{2-3}{2*3}=-\frac{1}{6}\\\\a_{1} +a_{2}+a_{3}=\frac{1}{2}+0+(-\frac{1}{6})=\frac{1}{2}-\frac{1}{6}=\frac{2}{6}=\frac{1}{3}\\\\Otvet:\boxed{\frac{1}{3}}$

$70)a_{n}=\frac{1-n}{2n}\\\\a_{1}=\frac{1-1}{2*1}=0\\\\a_{2}=\frac{1-2}{2*2}=-\frac{1}{4}\\\\a_{3}=\frac{1-3}{2*3}=-\frac{2}{6}=-\frac{1}{3} \\\\a_{1}+a_{2}+a_{3}=0+(-\frac{1}{4})+(-\frac{1}{3} )=-\frac{1}{4}-\frac{1}{3}=-\frac{7}{12}\\\\Otvet:\boxed{-\frac{7}{12}}$

Answer 2

Объяснение:

69) а1=(2-1)/(2×1)=1/2

   а1=(2-2)/(2×2)=0

а1=(2-3)/(2×3)=-1/6

1/2+0-1/6=(3-1)/6=2/6=1/3

70) а1=(1-1)/(2×1)=0

а2=(1-2)/(2×2)=-1/4

а3=(1-3)/(2×3)=-2/6=-1/3

0+(-1/4)+(-1/3)=(-3-4)/12= -7/12