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(*         *   The Coq Proof Assistant / The Coq Development Team       *)
(*  v      *   INRIA, CNRS and contributors - Copyright 1999-2018       *)
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Local Open Scope R_scope.
Proof with trivial.
apply Rsqr_inj...
apply cos_ge_0...
left; apply (Rlt_trans (- (PI / 2)) 0 (PI / 4) _PI2_RLT_0 PI4_RGT_0)...
left; apply PI4_RLT_PI2...
left; apply (Rmult_lt_0_compat 1 (/ sqrt 2))...
prove_sup...
apply Rinv_0_lt_compat; apply Rlt_sqrt2_0...
rewrite Rsqr_div...
rewrite Rsqr_1; rewrite Rsqr_sqrt...
unfold Rsqr; pattern (cos (PI / 4)) at 1;
rewrite <- sin_cos_PI4;
replace (sin (PI / 4) * cos (PI / 4)) with
(1 / 2 * (2 * sin (PI / 4) * cos (PI / 4))) by field.
rewrite <- sin_2a; replace (2 * (PI / 4)) with (PI / 2) by field.
rewrite sin_PI2...
field.
left; prove_sup...
apply sqrt2_neq_0...
Qed.

Lemma cos_3PI4 : cos (3 * (PI / 4)) = -1 / sqrt 2.