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6. (15 pts) For two protons a and b located at a (-10, 0,5) cm and b (20, 15, 10) cm relative to the center of a scanner running at 0.5 Tesla. Please calculate the resonance frequencies for a and b when: (Please get the results to the precision of Hz) 1) No magnetic gradient field is applied; 2) When there is only x gradient turned on with a strength of 2 G/cm; 3) When both y and z gradients are turned on with a strength of -4 G/cm