a. 8−215−8+215
= 3−215+5−3+215+5
= (3−5)2−(3+5)2
= 5−3−3−5
= −23
b. 3−115−5+25−45−25
= 2(15−5).(3+1)+4(5−25).(25+4)
=245+15−15−5+4(5−25).2(5+2)
= 235−5+2(5−25).(5+2)
= 225+255+10−10−45
= 5+25
= 235
c. (5−21+5+21):(2+1)21
= (5−2).(5+2)5+2+5−2.(2+1)2
= 325.(2+22+1)
= 325.(3+22)
= 365+410
d. (3−12+3−23+3−315).3+51
= (3+1−3(3+2)+25(3+3)).3+51
= (3+1−6−33+215+53).3+51
= (−23−5+215+53).3+51
= 2−43−10+15+53.3+51
= 23+5.3+51
= 21
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