Uzasadnij, że jeśli: a+b=1, a kwadrat + b kwadrat=7 to a do4 + b do4= 31
a + b = 1
a^2 + b^2 = 7
to
a^4 + b^4 = 31
Mamy
b = 1 - a
więc
a^2 + b^2 = a^2 + ( 1 - a)^2 = a^2 + 1 - 2a + a^2 = 2 a^2 - 2a + 1
2 a^2 - 2 a + 1 = 7
2 a^2 - 2a - 6 = 0 / : 2
a^2 - a - 3 = 0
delta = ( -1)^2 - 4*1*( -3) = 1 + 12 = 13
p (delty) = p(13)
a = ( 1 - p(13))/ 2 lub a = ( 1 + p(13))/2
---------------------------------------------------------
b = 1 - [ 0,5 - 0,5 p(13) ] = 0,5 + 0,5 p(13)
lub
b = 1 - [ 0,5 + 0,5 p(13) ] = 0,5 - 0,5 p(13)
dlatego
a^2 = ( 0,5 - 0,5 p(13))^2 = 0,25 - 0,5 p(13) + 13/4 = 7/2 - 0,5 p(13)
a^4 = 12,25 - 3,5 p(13) + 13/4 = 15,5 - 3,5 p(13)
oraz
b^2 = ( 0,5 + 0,5 p(13))^2 = 0,25 + 0,5 p(13) + 13/4 = 3,5 + 0,5 p(13)
b^4 = 12,25 + 3,5 p(13) + 13/4 = 15,5 + 3,5 p(13)
zatem
a^4 + b^4 = 15,5 - 3,5 p(13) + 15,5 + 3,5 p(13) = 31
Podobnie można sprawdzić dla
a = ( 1 + p(13))/2 i b = 0,5 - 0,5 p(13)
ckd.
============================================
p(13) - pierwiastek kwadratowy z 13
a²+b² = 7 I()²(
(a²+b²)² = 7²
(a²)²+2·a·b+(b²)² = 49
a⁴+2a²·b²+b⁴ = 49
a⁴+2(ab)²+b⁴ = 49
a+b = 1 I()²
(a+b)² = 1²
a²+2ab+b² = 1
a²+b² = 7
7+2ab = 1
2ab = 1-7
2ab = -6 /:2
ab = -3
a⁴+2(-3)²+b⁴ = 49
a⁴+2*9+b⁴ = 49
a⁴+b⁴ = 49-18
a⁴+b⁴ = 31
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c.n.d
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a + b = 1
a^2 + b^2 = 7
to
a^4 + b^4 = 31
Mamy
b = 1 - a
więc
a^2 + b^2 = a^2 + ( 1 - a)^2 = a^2 + 1 - 2a + a^2 = 2 a^2 - 2a + 1
2 a^2 - 2 a + 1 = 7
2 a^2 - 2a - 6 = 0 / : 2
a^2 - a - 3 = 0
delta = ( -1)^2 - 4*1*( -3) = 1 + 12 = 13
p (delty) = p(13)
a = ( 1 - p(13))/ 2 lub a = ( 1 + p(13))/2
---------------------------------------------------------
więc
b = 1 - [ 0,5 - 0,5 p(13) ] = 0,5 + 0,5 p(13)
lub
b = 1 - [ 0,5 + 0,5 p(13) ] = 0,5 - 0,5 p(13)
dlatego
a^2 = ( 0,5 - 0,5 p(13))^2 = 0,25 - 0,5 p(13) + 13/4 = 7/2 - 0,5 p(13)
a^4 = 12,25 - 3,5 p(13) + 13/4 = 15,5 - 3,5 p(13)
oraz
b^2 = ( 0,5 + 0,5 p(13))^2 = 0,25 + 0,5 p(13) + 13/4 = 3,5 + 0,5 p(13)
b^4 = 12,25 + 3,5 p(13) + 13/4 = 15,5 + 3,5 p(13)
zatem
a^4 + b^4 = 15,5 - 3,5 p(13) + 15,5 + 3,5 p(13) = 31
Podobnie można sprawdzić dla
a = ( 1 + p(13))/2 i b = 0,5 - 0,5 p(13)
ckd.
============================================
p(13) - pierwiastek kwadratowy z 13
a²+b² = 7 I()²(
(a²+b²)² = 7²
(a²)²+2·a·b+(b²)² = 49
a⁴+2a²·b²+b⁴ = 49
a⁴+2(ab)²+b⁴ = 49
a+b = 1 I()²
(a+b)² = 1²
a²+2ab+b² = 1
a²+b² = 7
7+2ab = 1
2ab = 1-7
2ab = -6 /:2
ab = -3
a⁴+2(-3)²+b⁴ = 49
a⁴+2*9+b⁴ = 49
a⁴+b⁴ = 49-18
a⁴+b⁴ = 31
--------------
c.n.d