Question

Find sum of first 12 terms of AP 2, 7, 12,
4
8 ਦੇ ਪਹਿਲੇ 15 ਗੁਣਜਾਂ ਦਾ ਜੋੜਫਲ ਪਤਾ ਕ​

Answer 1

sum of n terms (Sn) = n/2{ 2a + (n -1)d }

(1) a = 2 , d = 7 - 2 = 5 and n = 10

S10 = 10/2{ 2×2 + (10 -1)× 5}

= 5 { 4 + 45}

= 5 × 49

= 245

(ii) a = -37 , d = 4 and n = 12

S12 = 12/2{ -37×2 + (12-1)×4}

= 6{ -74 + 44}

= -180

(iii) a = 0.6 , d = 1.1 and n = 100

S100 = 100/2{ 2 × 0.6 + (100-1)×1.1 }

= 50 × { 1.2 + 108.9}

= 50 × 110.1

= 5505

(iv) a = 1/15 , d = 1/12 - 1/15 = 1/60

S11 = 11/2{ 2/15 + 10 × 1/60 }

= 11/2 { 18/60 }

= 33/20