znajdz x spejson: 3+7+11+...+x=300

:

3+x		x−3
	*(		+1)=300
2		4

3+x		x+1
	*		=300
2		4

x²+4x+3=2400 x²+4x−2397=0 x=−51 ∨ x=47

Janek191: a₁ = 3 r = 4 a_n = x S_n = 300 a_n = a₁ + (n−1)*r = 3 +( n −1)*4 = 4 n − 1 S_n = 0,5*(a₁ + a_n)*n = 0,5*(3 + 4 n − 1)*n= ( 1 + 2n)*n = n + 2 n² = 300 2 n² + n − 300 = 0 Δ = 1 − 4*2*(−300) = 2 401 √Δ = 49

	− 1 + 49
n =		= 12
	4

więc x = a_n = 4*12 − 1 = 47 =====================