Firstly finding common difference of the series
As we know that
• Common difference ( d ) = t₂ - t₁
→ Common difference = 37 - 40
→ Common difference = - 3
Now, as we know that
• Sn ( Sum of n terms ) = n/2 [ 2a + ( n - 1 ) d ]
Substituting n = 1. Since the sum of no. of terms is 10 . And substituting a = 40 ,since first term is 40
=> S₁₀ = 10/2 [ 2(40) + ( 10 - 1 ) -3 ]
=> S₁₀ = 5 [ 80 - 27 ]
=> S₁₀ = 5 × 53
=> S₁₀ = 265
Hence , sum of first 10 terms ( S₁₀ ) = 265
