Найдите сумму 7-ми первых прогрессии : 5, 10,20

Question

Алгебра: Найдите сумму 7-ми первых прогрессии : 5, 10,20

999Милкивейка999 · Accepted Answer

1) 4b(b+9)-3(b+12b)-b+7 = b+7
4b² + 36b - 3b - 36b - b + 7 - b - 7 =0
4b² - 5b = 0
b(4b - 5) = 0
b1 = 0
b2 = 5/4 = 1.25
2) 4b(b+9)-3(b+12b)-b+7 = 0
4b² + 36b - 3b - 36b -b +7 = 0
4b² - 4b +7 = 0
4b(b-1) + 7 =0
1. 4b + 7 =0
4b = -7
b1 = -1.75
2. b - 1 + 7 = 0
b +6 =0
b2 = -6
3) 4b(b+9)-3(b+12b)-b+7 = 7
4b² + 36b - 3b - 36b -b +7 - 7 = 0
4b² - 4b =0
4b (b - 1) = 0
b1 = 0
b2 = 1
4) 4b(b+9)-3(b+12b)-b+7 = 5
4b² + 36b - 3b - 36b -b +7 - 5 =0
4b² - 4b + 2 =0
2b² - 2b + 1 = 0
2b (b - 1) +1 = 0
1. 2b +1 = 0
b1 = -0.5
2. b - 1 + 1 =0
b2 = 0

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