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The main difference between let and lambda is that let is used to bind a value to a variable for use within a lexical scope, while lambda is used to define an anonymous function that can be used as a value.

When generating intermediate results in Common Lisp, the choice between let and lambda depends on whether the result is a simple value or a more complex set of operations.

Let is typically used when generating a simple value that is calculated within a local scope, such as converting a number to a string or performing a mathematical calculation. For example:

(let ((x 10))
  (+ x 5))

In this example, the value of x is bound to 10 within the let expression, and the result of adding 5 to x is returned as the intermediate result.

Lambda, on the other hand, is used when generating a more complex set of operations that can be reused multiple times within a larger program. For example:

(lambda (x)
  (let ((y (* x x)))
    (+ y 1)))

In this example, a lambda expression is used to define an anonymous function that takes an input value x and calculates y as the square of x. The intermediate result returned by the function is then the sum of y and 1.

Overall, the choice between let and lambda depends on the specific requirements of the program and the level of complexity involved in generating the intermediate results.