**1. The problem statement, all variables and given/known data**

There is a mass "m" on an incline plane of mass "M" with angle "θ". At what is the acceleration required of the block to keep the mass from sliding down?

**2. Relevant equations**

We did this problem using the lagrange approach and got an answer of x"=[mgsinθcosθ]/[M+m(sinθ)^2], but now we are supposed to do it using F=ma, and I can’t get that expression.

**3. The attempt at a solution**

Setting up the force diagram and using the parallel alternate interior angles thing from geometry, I get that:

cosθ=Fx/mgsinθ so,

Fx=mgsinθcosθ

But then Fx=(M+m)a, so I get:

a=[mgsinθcosθ]/[M+m]

Where does the sin^2 come into play?

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