设圆锥底面半径为r,r=x*R,可以算出x=1-a/2π
圆锥的容积v为πr²*(R²-r²)&sup0.5
v=πx²*R²*（R²-x²*R²）&sup0.5
v=πR³*（x&sup4-x&sup6）&sup0.5
对变量x&sup4-x&sup6求导得v'=4x³-6&sup5
令v’=0 求得x=6&sup0.5/3
再根据x=1-a/2π就可以求得a