But in symmetric AP, if $ d > 0 $, $ a - 3d $ could be negative. To make sense, assume $ a $ large. But both signs give negative — so only one is positive? Try numerically: $ \sqrt7 \approx 2.6458 $. Then $ a - 3d \approx a - 5.29 $, $ a + 3d \approx a + 7.97 $, $ a + d \approx a + 2.6458 $. For first term positive, $ a > 5.29 $; third term positive. Then $ \fraca + da - 3d - go-checkin.com