Why "Expression cannot begin with xxx " always show up?

Questions
1

Hi there,

I got a problem with solving equations using Mathematica recently. I typed in Solve[an equation], it always show up “Expression cannot begin with xxx”. (As shown in the image attached)

Does anyone know how to deal with it?

Thanks!

2

You’re using […] where you should be using (…)

[ ] should contain function arguments.

3

Which [] do you mean?

4

The [ ] pairs around BesselJ. Also, you don’t need Solve.

In[3]:= Integrate[(BesselJ[0, a*r])^2*r, {r, 0, R}]

Out[3]= 1/2 R^2 (BesselJ[0, a R]^2 + BesselJ[1, a R]^2)
5
6

I figured it out! Thanks for everyone!

Best,