Diket. barisan aritmetika dg U3=21 & U7+U8=69. Tentukan U15 Tolong di jawab yaaa,,,, bsk dikumpul

U3 = 21
U7+U8 = 69
U3 = a+2b = 21
U7 = a+6b
U8 = a+7b
》(a+6b) + (a+7b) = 69
2a + 13b = 69
Terus eliminasi antara U3 = 21 dan U7+U8 = 69
》(2a + 13b) - (a+2b) = 69 - 21
》(2a + 13b) - 2(a+2b) = 69 - 2(21)
》(2a + 13b) - (2a+4b) = 69 - 42
》(2a-2a) + (13b-4b) = 27
》9b = 27 》b = 27/9 》b = 3
# a+2b = 21
# a+2(3) = 21
# a+6 = 21 》a = 21-6 》a = 15
Dit: U15 ?
》U15 = a + (n-1)b
》U15 = 15 + (15-1)3
》U15 = 15 + (14)3 》U15 = 15 + 42 = 57