Browse · MathNet Print → Bulgarian Winter Tournament Bulgaria algebra Problem Find A2024, where An=1⋅2+3⋅4+5⋅8+⋯+(2n−1)⋅2n. Solution — click to reveal Since 2An=1⋅4+3⋅8+⋯+(2n−3)⋅2n+(2n−1)⋅2n+1, it follows An=2An−An=(2n−1)⋅2n+1−(1⋅2+2⋅4+2⋅8+⋯+2⋅2n)=(2n−1)⋅2n+1−2⋅(2+4+8+⋯+2n)+1⋅2=(2n−1)⋅2n+1−2⋅2⋅2−12n−1+2=(2n−3)⋅2n+1+6. Therefore A2024=4045⋅22025+6. Final answer 4045 * 2^2025 + 6 Techniques Sums and productsTelescoping series ← Previous problem Next problem →