I have attached an image of the question and solution but I need an explanation

Question

I have attached an image of the question and solution but I need an explanation of what was done.

I do not understand the multiplication of the 2nd trig function..

Why is it -120ACos20Cos20 and not just -120ACos20?

Where does the 2nd Cos 20 come front

For the given stale of stress, determine the normal and shearing stresses exerted on the oblique face of the shaded triangular element shown. Use a method of analysis based on the equilibrium of that element, as was done in the derivations of Sec. 7.2. delta A - 120A cos 20 degree cos20 degree - 45A cos20 degree sin 20 degree - 45A sin 20 degree cos20 degree - 60A sin 20 degree sin 20 degree = 0 delta = 120 cos 20 degree + 45 cos20 degree sin 20 degree + 45 sin 20 degree cos20 degree + 60A sin2 20 degree = 14.19 MPa + F = 0 tau A + 120A cos 20 degree sin20 degree - 45A cos20 degree cos 20 degree - 45A sin 20 degree sin 20 degree - 60A sin 20 degree cos 20 degree = 0 tau = - 120 cos 20 degree sin20 degree + 45(cos2 20 degree - sin2 20 degree) + 60 sin 20 degree cos 20 degree = 15.19 MPa

Explanation / Answer

Because you are summing only the component of forces that are aligned perpendicular to that oblique face.

The stress on the vertical face is 120 MPa.

The force on the vertical face is 120Acos20, because we are given that A is the area of the oblique face, therefore the area of the vertical face must be Acos20.

But because the angle between the vertical and oblique faces is 20, the force perpendicular to the oblique face has to be (120Acos20)(cos20).

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