∵a⊥b,∴a•b=0,即1×2+2x+3×4=0,解得x=-7.
故答案为-7.
— _____ are manatees?
— They are about 10 feet long and weigh about 1, 000 pounds. [ ]
A. How many
B. How much
C. How long
D. How big
肌肉的力量与耐力均衡吗?