In an a. p., if the 12th term is -13 and the sum of its first four terms is 24, find the sum of its first ten terms.

Since the progression is arithmetic, we have

So we have

Continuing the pattern above, we also find that

Writing and substituting into the first equation, we have

Now the sum of the first ten terms is

2.2°  

step-by-step explanation:

let θ = the angle of elevation.

tanθ =77/2020

tanθ = 0.038 12

      θ = tan⁻¹0.038 12

      θ = 2.2°