The FUNC function would have to have a body like return ( x*bhat + tinv(0.975, 22)*sqrt(x*inv*x`) - 0.3 );   /* find the zero of this function */   However, the equation you wrote is a multivariate function. Your original picture indicates that x is a scalar value, but In your equation, x would have to be an 8-dimensional vector.  The locus of x values for which the equation is satisfied is a 7-dimensional surface, which cannot be easily found.  (For example, the 7-dimensional surface y=x1^2 + x2^2 intersects y=0.3 on the 1-dimensional ellipse x1^2 + x2^2=03.)   In other words, there are infinitely many values of x that satisfy your equation.     ... View more