[tex]\displaystyle\bf\\a_{1} =-2\\\\d=-3\\\\n=5\\\\S_{5} =?\\\\\\a_{5} =a_{1} +4d=-2+4\cdot (-3)=-2-12=-14\\\\\\S_{5} =\frac{a_{1} +a_{5} }{2} \cdot 5=\frac{-2-14}{2} \cdot 5=\frac{-16}{2} \cdot 5=-8\cdot 5=-40\\\\\\Otvet: \ S_{5} =-40[/tex]
Ответ:
- 40.
Объяснение:
В арифметической прогрессии
Sn = (2•a1 + d•(n-1))/2 • n
S5 = (2•a1 + d•(5-1))/2 • 5
S5 = (2•a1 + 4•d)/2 • 5
S5 = (a1 + 2d) • 5 = ( -2 + 2•(-3))•5 = - 8•5 = - 40.
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Answers & Comments
[tex]\displaystyle\bf\\a_{1} =-2\\\\d=-3\\\\n=5\\\\S_{5} =?\\\\\\a_{5} =a_{1} +4d=-2+4\cdot (-3)=-2-12=-14\\\\\\S_{5} =\frac{a_{1} +a_{5} }{2} \cdot 5=\frac{-2-14}{2} \cdot 5=\frac{-16}{2} \cdot 5=-8\cdot 5=-40\\\\\\Otvet: \ S_{5} =-40[/tex]
1
=−2
d=−3
n=5
S
5
=?
a
5
=a
1
+4d=−2+4⋅(−3)=−2−12=−14
S
5
=
2
a
1
+a
5
⋅5=
2
−2−14
⋅5=
2
−16
⋅5=−8⋅5=−40
Ответ:
- 40.
Объяснение:
В арифметической прогрессии
Sn = (2•a1 + d•(n-1))/2 • n
S5 = (2•a1 + d•(5-1))/2 • 5
S5 = (2•a1 + 4•d)/2 • 5
S5 = (a1 + 2d) • 5 = ( -2 + 2•(-3))•5 = - 8•5 = - 40.