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I am facing a question, have just started to read a chapter from Smoth Manifolds by Lee and having trouble to do this excercise

Let w be a left invariant k-form on a Lie group G. Show that dw is a left invariant (k+1)-form.

I have re-read the definations but I can't proceed anywhere, I am new members in this site,I don't know many rules, but if I violate any, please forgive me.

Gracias señor!

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If $\alpha$ is an invariant form and $g\in G$, denote $L_g$ the left translation, $L_g^*\alpha=\alpha$ implies that $d(L_g^*\alpha)=L_g^*d\alpha=d\alpha$ since the exterior derivative commutes with pullbacks.

Proving that the pullback map commutes with the exterior derivative