Type a math noun in the math riddle.
1, a million little brothers, stand beside me and equidistant from me, forming a circle-circle 2, offer a humble apology-sum 3, arm wrestling-strength 4, similarity-approximate value 5, don't cheat in the exam-true score 6, goodbye to mom-denominator 7, doctor's advice. 1 1, two sheep fight-right angle 12, completely cost-effective-logarithm 13, both sides count-score 14, legal costs-rational number. Yelling at each other-18, divorce by agreement-about 19, counting by hand-index 20, looking around-moving things.