Question

A= 4+2^2+2^3+2^4+...+2^20

Answer 1

2.A = 2^3 +2^3 +2^4 +...+ 2^21

2A - A = (2^3 +2^3 +2^4 + ... + 2^21) - (2^2 + 2^2 +2^3 +...+ 2^20)

A = 2^3 +2^3 +2^4 + ... + 2^21 -2^2 -2^2 -2^3 - 2^4 +...+ 2^20

A = 2^3 + 2^21 -2^2 -2^2

A = (2^3 -2^2 -2^2) +2^21

A = 2^21