Find the sum of all the terms in the arithmetic progression 180, 175, 170, \ldots, 25.
Solution
The arithmetic progression (AP) is given by the sequence: 180, 175, 170, \ldots, 25.
The first term \(a = 180\) and the common difference \(d = 175 - 180 = -5\).
The last term \(l = 25\).
To find the number of terms \(n\), use the formula for the last term of an AP: \(l = a + (n-1) \cdot d\).
Substitute the known values: \(25 = 180 + (n-1) \cdot (-5)\).
Simplify: \(25 = 180 - 5(n-1)\).
\(25 = 180 - 5n + 5\).
\(25 = 185 - 5n\).
\(5n = 185 - 25\).
\(5n = 160\).
\(n = \frac{160}{5} = 32\).
Now, use the formula for the sum of an AP: \(S_n = \frac{n}{2} (a + l)\).
Substitute the known values: \(S_{32} = \frac{32}{2} (180 + 25)\).
\(S_{32} = 16 \cdot 205\).
\(S_{32} = 3280\).
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