The sum of the first 21 terms of the progression, -18, -15, -12,...is?
For any AP,
Sn = n/2 (2a + (n-1) d)
So plug in your numbers.
-630
252 is the answer
find the sum of the 21 term of the progression -18-15-12 ?
The sum of the first 21 term of the progrestion- 18-15,-12--- is
I got 252 too I don't know if I am correct
To find the sum of the first 21 terms of an arithmetic progression, you can use the arithmetic series formula.
The formula to find the sum of the first n terms of an arithmetic progression is:
Sn = (n/2) * (a + l)
where Sn is the sum of the first n terms, n is the number of terms, a is the first term of the progression, and l is the last term of the progression.
In this case, the first term (a) is -18, and the last term (l) can be found by using the formula for the nth term of an arithmetic progression:
ln = a + (n-1)d
where ln is the nth term, d is the common difference between the terms, and n is the term number.
In this case, the common difference (d) is 3 since each term increases by 3. So, using the formula, we can find the 21st term:
l21 = -18 + (21-1)*3 = -18 + 20*3 = -18 + 60 = 42.
Now that we have the value of the last term, we can substitute it into the arithmetic series formula to find the sum:
S21 = (21/2) * (-18 + 42) = 21 * 24 = 504.
Therefore, the sum of the first 21 terms of the progression -18, -15, -12,... is 504.