Restate the following statements with appropriate conditions, so that they become true.
If , then .
If and , then .
If , then .
If and , then .
If , then .
If and , then .
The diagonals of a quadrilateral bisect each other.
The diagonals of a parallelogram bisect each other.
Explanation
This question tests the ability to identify missing conditions that make mathematical statements universally true. Each original statement has counterexamples that make it false. For (i), if a = -5 and b = 3, then a² > b² but a < b. Adding the condition that both numbers are positive makes it true. For (ii), x = 3 and y = -3 gives x² = y² but x y; restricting to non-negative numbers resolves this. For (iii), expanding gives 2xy = 0, so either x = 0 or y = 0; the condition y 0 ensures x must be 0. For (iv), not all quadrilaterals have bisecting diagonals—only parallelograms have this property.