Question

S11= 121 d= -3, найти a20

Answer 1

Ответ:

Объяснение:

S11=121    ,   d=-3   , a20=?

&

Sn=(2a1+d(n-1)/2  *n

S11=(2a1+d(11-1)/2 *11

121=(2a1-3*10)*5,5

121=11a1-165

11a1=165+121

11a1=286 // : 11

a1=26

an=a1+d(n-1)

a20=26-3(20-1)

a20=26-57

a20=-31

Answer 2

It looks like you're dealing with a sequence where the first term is S11 = 121, and the common difference between consecutive terms is d = -3. You're trying to find the value of the 20th term, which is denoted as a20.

In a sequence with a constant difference between terms, you can use the arithmetic progression formula to find the nth term:

$a_n = a_1 + (n - 1) \cdot d$

Where:

• $a_n$ is the nth term
• $a_1$ is the first term
• $d$ is the common difference between terms
• $n$ is the term number

In your case:

• $a_1$ is not given directly, but we have the 11th term $S11 = 121$
• $d = -3$
• $n = 20$

We need to find $a_1$ using the 11th term $S11$: $S11 = a_1 + (11 - 1) \cdot d$ $121 = a_1 + 10 \cdot (-3)$ $121 = a_1 - 30$ $a_1 = 151$

Now that we know $a_1 = 151$, we can find $a_{20}$: $a_{20} = a_1 + (20 - 1) \cdot d$ $a_{20} = 151 + 19 \cdot (-3)$ $a_{20} = 151 - 57$ $a_{20} = 94$

So, the 20th term $a_{20}$ is 94.