



 
Hi Tim, In the diagram below triangle ADC is a right triangle and the length of CD is (r  150) mm. Pythagoras theorem gives you \[1200^2 + (r  150)^2 = r^2\] Expanding the left side gives \[1200^2 + r^2  300 r + 150^2 = r^2\] Simplifying gives \[300 r = 1200^2 + 150^2 = 1462500\] and hence \[r = \frac{1462500}{300} = 4875 \mbox{ mm}\] Penny  


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