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a = 11-b
u3+u4= 2a+ 5b = 31
2a + 5b = 31
2(11-b) +5b = 31
22 - 2b +5b = 31
3b = 9
b=3
a = 11- b= 11-3=8
S10= 1/2x10( 2x8 + (10-1)3
=5(16+27)
= 215