Module: Mathlib.Algebra.ContinuedFractions.ConvergentsEquiv

Goal state before:
pp: case inr.succ
K : Type u_1
g : GenContFract K
inst✝ : Field K
a b : K
b_ne_zero : b ≠ 0
n' : ℕ
nth_partDen_ne_zero : ∀ {b : K}, g.partDens.get? (n' + 1) = some b → b ≠ 0
not_terminatedAt_n : ¬g.TerminatedAt (n' + 1)
s_nth_eq : g.s.get? (n' + 1) = some { a := a, b := b }
pa pb : K
s_n'th_eq : g.s.get? n' = some { a := pa, b := pb }
g' : GenContFract K := g.squashGCF (n' + 1)
pred_conts : Pair K := g.contsAux (n' + 1)
succ_n'th_contsAux_eq : pred_conts = g.contsAux (n' + 1)
ppred_conts : Pair K := g.contsAux n'
n'th_contsAux_eq : ppred_conts = g.contsAux n'
pA : K := pred_conts.a
pB : K := pred_conts.b
ppA : K := ppred_conts.a
ppB : K := ppred_conts.b
pred_conts' : Pair K := g'.contsAux (n' + 1)
succ_n'th_contsAux_eq' : pred_conts' = g'.contsAux (n' + 1)
ppred_conts' : Pair K := g'.contsAux n'
n'th_contsAux_eq' : ppred_conts' = g'.contsAux n'
pA' : K := pred_conts'.a
pB' : K := pred_conts'.b
ppA' : K := ppred_conts'.a
ppB' : K := ppred_conts'.b
this✝ : g'.convs (n' + 1) = ((pb + a / b) * pA' + pa * ppA') / ((pb + a / b) * pB' + pa * ppB')
this : g.convs (n' + 2) = (b * (pb * pA + pa * ppA) + a * pA) / (b * (pb * pB + pa * ppB) + a * pB)
⊢ (b * (pb * pA + pa * ppA) + a * pA) / (b * (pb * pB + pa * ppB) + a * pB) =
    ((pb + a / b) * pA' + pa * ppA') / ((pb + a / b) * pB' + pa * ppB')

Tactic:
suffices
        ((pb + a / b) * pA + pa * ppA) / ((pb + a / b) * pB + pa * ppB) =
          (b * (pb * pA + pa * ppA) + a * pA) / (b * (pb * pB + pa * ppB) + a * pB) by
        obtain ⟨eq1, eq2, eq3, eq4⟩ : pA' = pA ∧ pB' = pB ∧ ppA' = ppA ∧ ppB' = ppB := by
          simp [*, g', pA, pB, ppA, ppB, pA', pB', ppA', ppB',
            (contsAux_eq_contsAux_squashGCF_of_le <| le_refl <| n' + 1).symm,
            (contsAux_eq_contsAux_squashGCF_of_le n'.le_succ).symm]
        symm
        simpa only [eq1, eq2, eq3, eq4, mul_div_cancel_right₀ _ b_ne_zero]

Goal state after:
case inr.succ
K : Type u_1
g : GenContFract K
inst✝ : Field K
a b : K
b_ne_zero : b ≠ 0
n' : ℕ
nth_partDen_ne_zero : ∀ {b : K}, g.partDens.get? (n' + 1) = some b → b ≠ 0
not_terminatedAt_n : ¬g.TerminatedAt (n' + 1)
s_nth_eq : g.s.get? (n' + 1) = some { a := a, b := b }
pa pb : K
s_n'th_eq : g.s.get? n' = some { a := pa, b := pb }
g' : GenContFract K := g.squashGCF (n' + 1)
pred_conts : Pair K := g.contsAux (n' + 1)
succ_n'th_contsAux_eq : pred_conts = g.contsAux (n' + 1)
ppred_conts : Pair K := g.contsAux n'
n'th_contsAux_eq : ppred_conts = g.contsAux n'
pA : K := pred_conts.a
pB : K := pred_conts.b
ppA : K := ppred_conts.a
ppB : K := ppred_conts.b
pred_conts' : Pair K := g'.contsAux (n' + 1)
succ_n'th_contsAux_eq' : pred_conts' = g'.contsAux (n' + 1)
ppred_conts' : Pair K := g'.contsAux n'
n'th_contsAux_eq' : ppred_conts' = g'.contsAux n'
pA' : K := pred_conts'.a
pB' : K := pred_conts'.b
ppA' : K := ppred_conts'.a
ppB' : K := ppred_conts'.b
this✝ : g'.convs (n' + 1) = ((pb + a / b) * pA' + pa * ppA') / ((pb + a / b) * pB' + pa * ppB')
this : g.convs (n' + 2) = (b * (pb * pA + pa * ppA) + a * pA) / (b * (pb * pB + pa * ppB) + a * pB)
⊢ ((pb + a / b) * pA + pa * ppA) / ((pb + a / b) * pB + pa * ppB) =
    (b * (pb * pA + pa * ppA) + a * pA) / (b * (pb * pB + pa * ppB) + a * pB)