With gupta advancements in mathmatics and astronomy, which of the following discoveries were most important to future generations? mark all that apply. use of zero as a placeholder numeric decimal system observing the earth to be round due to shadows concluding the earth spins on an axis

1. 2. ∠b and ∠y are right angles. 3.? 4.? which two statements are missing in steps 3 and 4? ∠x ≅ ∠c △abc ~ △zyx by the sas similarity theorem. ∠b ≅ ∠y △abc ~ △zyx by the sas similarity theorem. = 2 △abc ~ △zyx by the sss similarity theorem. = 2 △abc ~ △zyx by the sss similarity theorem.