Triangles X Y Z and M N O are shown. The length of side X Y is 21 and the length of Y Z is 14. The length of side N O is 4 and the length of M O is 5. What additional information is needed to prove that the triangles are similar? To prove △XYZ ~ △MNO using SSS, you need to know that . Now that you know the length of XZ, you need to know that angle O is congruent to to prove △XYZ ~ △MNO using SAS.