effective duration and effective convexity to calculate price change

Steve Jacobs, CFA, is analyzing the price volatility of Bond Q. Q’s effective duration is 7.3, and its effective convexity is 91.2. What is the estimated price change for Bond Q if interestrates fall/rise by 125 basis points?

Estimated return impact if rates fall by 125 basis points:

≈ −(Duration × ΔSpread) + ½ Convexity × (ΔSpread)2
≈ −(7.3 × −0.0125) + ½(91.2)(0.0125)2
≈ +0.09125 + 0.007125
≈ +0.0983750
≈ +9.84%

Estimated return impact if rates rise by 125 basis points:

≈ −(Duration × ΔSpread) + ½ Convexity × (ΔSpread)2
≈ −(7.3 × +0.0125) + ½(91.2)(0.0125)2
≈ −0.09125 + 0.007125
≈ −0.084125
≈ −8.41%

公式里convexity 前面不需要乘1/2的， 为什么这里要乘1/2，求解惑。