f(x)=x²

fsquare.js

const forms = [
  () => {
    const limits = [...new Array(2)].map(()=>~~(Math.random()*40)-20)
    const symbols = ['<', '≤'].map((v,i,a)=>a[~~(Math.random()*a.length)])
    
    return `${Math.min(...limits)}${symbols[0]}x${symbols[1]}${Math.max(...limits)}`
  },
  () => {
    const symbols = ['<', '≤', '=', '≥', '>']
    return `x${symbols[~~(Math.random()*symbols.length)]}${~~(Math.random()*40)-20}`
  }
]

const value = forms[~~(Math.random()*forms.length)]

console.log(value())