# I need help checking an expression

**URL:** <https://cl.desmos.com/t/i-need-help-checking-an-expression/3962>\
**Category:** Questions\
**Tags:** patterns, validation, correctness\
**Created:** [January 15, 2022, 2:21am UTC](https://cl.desmos.com/t/i-need-help-checking-an-expression/3962 "2022-01-15T02:21:49Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![dpharo](https://avatars.discourse-cdn.com/v4/letter/d/c5a1d2/32.png) [@dpharo](https://cl.desmos.com/u/dpharo)\
**Post date:** [January 15, 2022, 2:21am UTC](https://cl.desmos.com/t/i-need-help-checking-an-expression/3962/1 "2022-01-15T02:21:49Z")

</div>

I have edited this great activity for a course I am teaching. On slides 6 and 7 the students input an expression to correct an error the machine makes. I have a working check on slide 6, but it won’t work on slide 7. Can someone help me with slide 7? Thanks so much.

[011-38 Formulas](https://teacher.desmos.com/activitybuilder/custom/61e068e825a09c0a39296303)

---

<div class="post-metadata">

**Author:** ![Daniel\_Grubbs](https://yyz1.discourse-cdn.com/flex031/user_avatar/cl.desmos.com/daniel_grubbs/32/719_2.png) [@Daniel\_Grubbs](https://cl.desmos.com/u/Daniel_Grubbs)\
**Post date:** [January 15, 2022, 2:43am UTC](https://cl.desmos.com/t/i-need-help-checking-an-expression/3962/2 "2022-01-15T02:43:44Z")

</div>

Maybe try rounding the key functions?

---

<div class="post-metadata">

**Author:** ![Elayna](https://yyz1.discourse-cdn.com/flex031/user_avatar/cl.desmos.com/elayna/32/2253_2.png) [@Elayna](https://cl.desmos.com/u/Elayna)\
**Post date:** [January 15, 2022, 6:02pm UTC](https://cl.desmos.com/t/i-need-help-checking-an-expression/3962/3 "2022-01-15T18:02:36Z")

</div>

I would use pattern matching, here. Something like this:

```auto
p= patterns
expr= p.radical(p.fraction(p.literal("V"),
                          p.product(p.literal("\pi"),p.literal("h")) ) )

correct = 
expr.matches(this.latex)

```

---

<div class="post-metadata">

**Author:** ![dpharo](https://avatars.discourse-cdn.com/v4/letter/d/c5a1d2/32.png) [@dpharo](https://cl.desmos.com/u/dpharo)\
**Post date:** [January 17, 2022, 3:05am UTC](https://cl.desmos.com/t/i-need-help-checking-an-expression/3962/4 "2022-01-17T03:05:21Z")

</div>

> [@Elayna](#):
>
> ```auto
> p= patterns
> expr= p.radical(p.fraction(p.literal("V"),
> p.product(p.literal("\pi"),p.literal("h")) ) )
> 
> correct = 
> expr.matches(this.latex)
> 
> ```

Thanks so much, this works.