玉米种子和大豆种子相比,大豆种子没有()
A:种皮
B:胚
C:胚乳
D:子叶
已知地球半径为R,质量为M,万有引力常量为G.一位质量为m的探险家乘坐热气球到达离地面h高处,他受到地球的万有引力大小为( )
① Are these all ours? ____ A. It is Zhang Peng's.
② Whose is it? ____ B. Yes,it is.
③ Is this yours?____ C. Yes. they are.
设x1和x2是方程x2+7x+1=0的两个根,则x12+x22= ____.