in an ap t3 =7 and t7=23. find S20
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740
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a3 = a+2d =7
a7 = a+6d =23
a + 2d= 7
-a - 6d =-23
-------------------
-4d = -16
d = 4
-------------------
a + 2d =7
a + 2(4)=7
a = -1
Sn = n/2 × {2a + (n-1)d}
S20= 20/2 × {2(-1) +(20-1)4}
= 10×{-2+76}
=10×74
=740.
S20 = 740.
a7 = a+6d =23
a + 2d= 7
-a - 6d =-23
-------------------
-4d = -16
d = 4
-------------------
a + 2d =7
a + 2(4)=7
a = -1
Sn = n/2 × {2a + (n-1)d}
S20= 20/2 × {2(-1) +(20-1)4}
= 10×{-2+76}
=10×74
=740.
S20 = 740.
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