Given that, a × b > 0 That means either a, b both are positive or both are negative. As, the point (a, b) lies in the third quadrant so we can say a, b both are negative. If a is negative then (–a) must be positive. Similarly, if b isnegative then (–b) must be positive. Now, (-a, -b) must lie in first quadrant (quadrant I) as both (–a & -b) are positive.