Question

5. Find the sum of 20 terms of the AP (x + y), (x - y), (x - 3y),...​

Answer 1

Given term = x+y , x-y , x-3y

a=(x+y)

d= a₂-a₁

 = (x-y - x - y)

 = -2y

Sn = n/2[2a+(n-1)d]

S₂₀=20/2[(2*x+y)+(22-1)-2y)

    = 10(2x+2y-42y)     

    = 10(2x-40y)                                   

    = 20x - 400y