Shouldn't there be mutual inductance only when current changes? The current is not changing here. Then how can we have mutual inductance.
Even if current how can I calculate mutual inductance without knowing the height of the triangle?
But on other thoughts, m12 = m21. But the triangular loop can not induce anything in the wire. Because it has 0 area and 0 flux. But if current changes, the wire can induce current in the triangular loop. Then how does m12=m21 stand in this case?