h=-5t^2+vt=-5[t^2-2(v/10)t+(v/10)^2]+v^2/20 =-5[(t-v/10)^2+v^2/衡纯20] 不配方也行,求一下-b/薯厅2a所以t=v/10的时候,h有最大值数拦隐为v^2/20 于是 15=v^2/20 v=10倍根号3= 17.32m/s