用托盘天平称1粒米的质量,较好的办法是()
A:认真仔细地直接测量
B:先称一粒米与一块木块的共同质量,再测木块质量,然后计算求得
C:先称10g质量的米.然后再数粒数,计算求得
D:先称10粒米的质量,然后计算求得
二次函数y=ax2+bx+c的图象如图所示,在ab、ac、b2﹣4ac,2a+b,a+b+c,这五个代数式中,其值一定是正数的有( )
某几何体的三视图如图所示,则这个几何体是
Microsoft ended support for Windows XP after April 8, which _______a major operating system for some Chinese computer users, and advised users to upgrade to Windows 8.1.