One rainy Thursday, he flipped to a random page. Problem 789: A father is three times as old as his son. In 12 years, he will be twice as old. Find their ages.
But by October, the culegere had become a symbol of failure. Problem 347: Solve the system of equations . He’d stare at the two innocent-looking lines until the x’s and y’s blurred. Problem 512: Study the monotonicity of the function . The arrows (↑ for increasing, ↓ for decreasing) felt like personal accusations. culegere matematica clasa a 9 a
He checked twice. No mistake. He checked the answer key at the back—it only said “Impossible. Explain why.” One rainy Thursday, he flipped to a random page
“The equations force the son to be 9 and the father 36, with sum 45. Since 45 is composite (3 × 15, 5 × 9), the condition ‘sum is prime’ cannot be met. Therefore, no such ages exist in whole numbers.” Find their ages
That night, he didn’t stop at three problems. He solved five. Then ten. By December, the blue culegere was battered but beloved. And when his teacher asked the class, “Who enjoys the challenge problems at the end of each chapter?” Andrei raised his hand.