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diff --git a/1172/CH6/EX6.12.1/6_12a.txt b/1172/CH6/EX6.12.1/6_12a.txt
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+++ b/1172/CH6/EX6.12.1/6_12a.txt
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+ # Problem 12a # 
+
+ Standard formula used 
+ u_x = (u_x_ + v) / (1 + v * u_x_ / c^2)
+ Velocity of photon with respect to another is -2.769231e+08 m/s.
+ Relativistic mass of particle with respect to another is 7.800000e-25 kg.
diff --git a/1172/CH6/EX6.12.1/Example6_12a.sce b/1172/CH6/EX6.12.1/Example6_12a.sce
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+++ b/1172/CH6/EX6.12.1/Example6_12a.sce
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+clc
+//Given that
+c = 3e8 // speed of light in m/s
+u_x_ = -2e8 // velocity of first photon in ground frame in m/s
+v = -2e8 // velocity of second photon in ground frame in m/s
+m_0 = 3e-25
+// sample problem 12 page No. 226
+printf("\n \n\n # Problem 12a # \n")
+printf("\n Standard formula used \n u_x = (u_x_ + v) / (1 + v * u_x_ / c^2)")
+u_x = (u_x_ + v) / (1 + v * u_x_ / c^2) // calculation of Velocity of photon with respect to another
+m = m_0 / sqrt(1 - (u_x / c)^2) // calculation of Relativistic mass of particle with respect to another
+printf ("\n Velocity of photon with respect to another is %e m/s.",u_x)
+printf ("\n Relativistic mass of particle with respect to another is %e kg.",m)