First, you make a force body diagram to illustrate the problem. Then, apply Newton's laws of motion.
Summation of forces along the y-direction:
F = 0 (bodies at equilibrium) = Normal force - weight*cos45 = 0
Normal force = wcos45
Summation of forcesalong the x-direction:
F = 0 (bodies at equilibrium) = Frictional force - weight*sin45 = 0
x*Normal force - weight*sin45 = 0 (let x be the min coeff of static friction)
x*weight*cos45 = weight*sin 45
x = sin45/cos45 = 1
Therefore, the minimum coefficient of static friction must be 1.
