Example 6 2 Cardiac output is an important factor in athletic endurance. The graph shows a stress-test graph of cardiac output (measured in litres/min of blood) versus workload (measured in kg m/min). cardiac A a Estimate the average rate of change output of cardiac output with respect to (litres/min) workload as the workload increases 20 from 0 to 1200 kg m/min. 15 b Estimate the instantaneous rate 10 of change of cardiac output with respect to workload at the 5 point where the workload is 450kg m/min. 0 300 600 900 1200 1500 workload (kg m/min)