protected static boolean isFirstDegreeExpression(Expression expr) {
		if(expr.length() != 2 || !expr.sign() || expr.exponent() != 1) return false;
		
		AlgebraicParticle a = expr.get(0), b = expr.get(1);
		//if one is a variable or variable with exponent, and the other is a number/mixed number/simplified fraction
		if(Util.constant(a) && b instanceof Term && isVariableWithOptionalCoefficient((Term)b)) return true;
		if(Util.constant(b) && a instanceof Term && isVariableWithOptionalCoefficient((Term)a)) return true;
		else return false;
	}