T2EE ‐ Corrigé du devoir en classe de mathématiques III,1 Exercice 1
a) fx 3x4−2x3 5
2x2 −x2 3 Df Df′ R
∀x ∈ Df′ : f′x 34x3−23x2 5
2 2x−1 12x3−6x25x−1
b) fx −57x2447 Df Df′ R
∀x ∈ Df′ : f′x −547x24472x −280x7x243
c) fx −3
5x22 −35x2−2 condition: 5x2≠ 0 x ≠ −2
5 Df Df′ R∖ −2
5
∀x ∈ Df′ : f′x −3 −25x2−3 5 30
5x23
d) fx 52x 1−3x
condition: 1−3x ≠ 0 x ≠ 1 3 Df Df′ R∖ 1
3
∀x ∈ Df′ : f′x 21−3x−52x−3
1−3x2 2−6x156x
1−3x2 17
1−3x2
e) fx
u
2x2
v
x1
condition: x1≥ 0 x ≥ −1 Df −1; Df′ −1;
∀x ∈ Df′ : f′x
u′
4x
v
x1
u
2x2
v′
1
2 x1 4x x1 x2 x1
4xx1x2
x1 4x24xx2
x1 5x24x x1
f) fx x2 −4x−21 condition: x2 −4x−21 ≥ 0 Δ 1684 100; x1 410
2 7; x2 4−10 2 −3
x − −3 7
x2−4x−21 0 − 0
Df −;−37;et Df′ −;−37;
∀x ∈ Df′ : f′x 2x−4
2 x2−4x−21 2x−2
2 x2−4x−21 x−2 x2−4x−21
g) fx 5 sin2x−3 sin x Df Df′ R
∀x ∈ Df′ : f′x 52 sin xcos x−3cos x 10 sin x cos x3 cos x cos x10 sin x3
h) fx
u
x2−x
v
x condition: x 0 Df Df′ 0;
∀x ∈ Df′ : f′x
u′
2x−1
v
x −
u
x2−x
v′
1 2 x
v2
x
22x−1x− x2−x 2 x
x 4x2−2x−x2x
2x x 3x2−x 2x x
i) fx
u
3x43
v
5−4x4 Df Df′ R
∀x ∈ Df′ : f′x
u′
33x423
v
5−4x4
u
3x43
v′
45−4x3 −4
93x425−4x4−163x435−4x3
3x425−4x395−4x−163x4
3x425−4x345−36x−48x−64
3x425−4x3−84x−19
j) fx −3 cos4−5x6
Df Df′ R
∀x ∈ Df′ : f′x −34cos3−5x6 −sin−5x6 −5 −60 cos3−5x6sin−5x6