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devchenka1998
@devchenka1998
August 2022
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как найти а1 и аn в арифметической прогрессии,если d=5 n=10 Sn=385.Помогите пожалуйста...
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2407Alexa
Verified answer
Аn=a1+d(n-1)
Sn=((a1+an)/2)*n
n=10
d=5
S(10)=385
a1=an-d(n-1)=a10-d(10-1)=a10-9d
S(10)=((a1+a10)/2)*10=(a1+a10)*5
a1+a10=S(10)/5
a10-9d+a10=385/5
2a10-9*5=77
2a10=77+45=122
a10=122/2
a10=61
a1=a10-9d=61-9*5=61-45
a1=16
S(10)=((a1+a10)/2)*10
385=(16+61)/2)*10
385=(77/2)*10
385=38,5*10
385=385-истино
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Answers & Comments
Verified answer
Аn=a1+d(n-1)Sn=((a1+an)/2)*n
n=10
d=5
S(10)=385
a1=an-d(n-1)=a10-d(10-1)=a10-9d
S(10)=((a1+a10)/2)*10=(a1+a10)*5
a1+a10=S(10)/5
a10-9d+a10=385/5
2a10-9*5=77
2a10=77+45=122
a10=122/2
a10=61
a1=a10-9d=61-9*5=61-45
a1=16
S(10)=((a1+a10)/2)*10
385=(16+61)/2)*10
385=(77/2)*10
385=38,5*10
385=385-истино