sont/est?

[Sunshine]

Do we say

A mon avis je pense que les maths sont interessants

or

A mon avis je pense que les maths est interessant

or

A mon avis je pense que les maths c'est interessant

and why please?

[George Hunt]

Well I think you either say "A mon avis..." or "Je pense que....." since both mean the same thing (in my opinion-I may be being pedantic).

Also :
A mon avis (je pense que ) les maths est interessant is incorrect since "les maths" is in the plural.

Both of your other alternatives seem acceptable to me.

Hoping that I am correct...