when 12ohm resistance joint in triangle caculate 1arm resistance

Your question is illegible. A figure is need to explain what you are talking about.

To calculate the equivalent resistance (1arm resistance) of a triangle circuit with three 12-ohm resistors, you can use the formula:

1/R = 1/R1 + 1/R2 + 1/R3

In this case, R1 = R2 = R3 = 12 ohms since all three resistors are of the same value. Substituting these values into the formula, we get:

1/R = 1/12 + 1/12 + 1/12
= 3/12
= 1/4

Now, to find the 1arm resistance, we need to invert the result:

R = 1/(1/4)
= 4 ohms

Therefore, the 1arm resistance of the triangle circuit with three 12-ohm resistors is 4 ohms.