7). а₁ = ? S₁₅ = 375 и d = 3.
Решение.
S₁₅ = (2a₁ + 14d)*15/2;
375 =(2a₁ + 14*3) * 15/2
450 = (2a₁ + 52) * 15
30 = 2a₁ + 52
2a₁ = -22
a₁ = -11
8). b₂=125, b₄= 5. S₅=?
Решение .
a) b₃² = b₂* b₄
b₃² = 5*125
b₃² = 625
b₃ = +-25
б) b₃ = 25 в) b₃ = -25
625; 125; 25; 5 ;... q = 1/5 -625; 125; -25; 5 ; ... q = -1/5
S₅ = b₁ (q⁵ -1)/(q -1) S₅ = b₁ (q⁵ -1)/(q -1)
S₅ = 625* ((1/5)⁵ -1)/(1/5 -1) S₅ =- 625* ((-1/5)⁵ -1)/(-1/5 -1)
S₅ = 781 S₅= 521
б) f'(x) = -1/4cos²x
в) f'(x) = -2x/sin²x
г) f'(x) = 2cosx+1/cos²x